Common Causes of Flooding on Reef Tanks

When your box full of water isn’t anymore

Just about anyone with a fish tank is familiar with spilling water at some point or another (it happens to the best of us!) only a few have experienced a flood. While I have had to do an emergency tank teardown to fix a leaking bulkhead, I have never had a full flood and intend to keep it that way. With a pragmatic approach to set our tanks up right, we can limit the possibilities of having the worst happen.


Potential Causes of Floods

Unintended Siphoning from the ATO Reservoir

On the line that pumps from the reservoir to either the tank or the Sump, be aware that a siphon effect will occur if the outlet of the line is submerged. To avoid this, have the outlet tubing drop the water into the sump from a high level, or use a small “anti-siphon” hole in the line to prevent this. You can also use a Peristaltic Pump as your ATO pump, which are impervious to siphoning of any kind, but their flow rates are much lower than conventional pumps.

Always take great care to securely fasten any hoses or tubing to the right place, as a dislodged hose can result in emptying of the reservoir in short order. Keeping an eye on your Reef Log as well will make sure that you know when it’s time to fill up your reservoir.


Hang-on Overflow setups

Overflow setups are one thing that can go horribly wrong if they are not set up right. Personally, I don’t know how anyone sleeps at night if they’re using one of those “over the tank” type overflows. They rely on airpumps to maintain a siphon which inherently makes them less secure than overflow that works with gravity alone. Having your return pump constantly pumping water up to your tank means all that water needs a place to go. The overflow is probably the place where it’s most crucial to plan out your setup and have failsafes in place. see Herbie Overflow Guide


Having Insufficient Sump Capacity

Some beginners will make the mistake of not designing their sump systems to accommodate enough volume in their sump for when the return pump gets turned off. Figuring this stuff out when you are doing a leak test in your garage or outdoors can save you later. Proper Sump Design is Covered here: Reef Aquarium Sump Tank Design



ReactorMany reactors operate with a combination of small fittings and higher pressure, so leaks are common. Even a small pump has the ability to remove a lot of water from your system in a short time. Risk can be minimized by hanging your reactor from the inside edge of the glass of the sump, so any potential leaks will be contained there as opposed to removing water from the system if located somewhere else in the cabinet.

This can also be dependent on the quality of the Reactor. While some of the more expensive models are built with tight fitting, water-tight hardware, many of the cheaper ones have very flimsy rubber fittings. An extra Zip-Tie or Hose Clamp on any barbed fitting is good insurance against the chance of a hose coming loose.


Plumbing lines that are not glued or union’d

Ideally, everything should be glued or secured with a union in your return line and drains. The only place where you can get away with not gluing fittings is inside your overflow. Learn more about the best practices of assembling fittings here: How to Measure, Cut and Glue PVC Pipe

Float switch Malfunctions

Float switches have been a staple of the hobby since the 90's. They are slowly being replaced by air sensors or optical sensors to trigger equipment to run in reef systems.

Getting something stuck in the mechanism – Care should be taken to keep things away from float switches. Aside from pesky snails which may prevent the switch from working, a small piece of debris can get lodged inside the switch. This can be something as innocuous as a single grain of carbon.

Small Wires – float switches use small, low-voltage wire running between the sensors and main unit. These wires can become crushed, stretched or cut off completely — breaking the electrical connection. Prevent this by tucking wires away from moving parts like cabinet doors or places where they can get tugged on. They won't stand up to much abuse.


Tank or Sump Failures

A pane of glass with a hole in it is weaker than one without. In this hobby, drilling holes is commonplace, but it can be easy to forget what you are doing to the strength of the glass.

Plumbing Stresses

A bulkhead in a glass hole can be the vulnerable stress point of a tank. When you attach pipe to the bulkhead, you create a Lever.

If enough force is applied to the lever, the fulcrum (the bulkhead) will cause the glass around it to crack. This is more common with thinner glass, but it can happen to any size tank. People have cracked their tanks from being too rough with their plumbing by pushing or pulling too hard on a pipe.

Take care to secure your plumbing to the stand where possible, and be careful not to exert force on bulkheads or pipe when moving a tank.

Faulty Acrylic seams or silicone joints in the tank are usually uncommon but worth mentioning. Always leak test your tanks (even new ones) and carefully inspect seams. When buying a tank, always make sure that it is an aquarium with sufficiently thick glass and not a terrarium or reptile tank.  An old aquarium might need to be resealed. Here is a great video on YouTube:

Traditional braced aquariums – wicking through the tank rim.

One thing to be aware of is something called capillary action. It is also called wicking. The way that tank rims are siliconed onto the rim of the tank means that there is a tiny gap between the plastic and the glass. Due to the surface tension qualities of water, water in these tiny gaps can travel up and over the rim of the tank against gravity.

This wicking can only happen if you fill the tank up too much. If you are topping up a tank, don’t let the water touch the plastic of the tank rim. Otherwise you could be looking at a leak, even though there is no crack in the tank.


$10 Insurance – Water Alarm

You can pick up a cheap alarm designed for basement sump systems for many home improvement store for around $10.00. This will create a loud piercing alarm when it detects water. Place in your cabinet where water will collect if spilled out the sump. To do this, you can design your cabinet to make your sump sit in a “dish” to ensure water will touch the sensor before running out onto the floor. At least then if someone is home they can bring it to your attention quickly.

The stand that the sump is inside of plays a part in how it can deal with any leaks. For more info on stand design see this article: Reef Tank Stands – A design Guide


Prepare for the worst by being ready to handle a tank teardown

If there is a major issue with your tank – that can mean it needs to be drained completely. Being properly prepared for that occurrence can mean the difference between losing all your livestock versus bouncing back within hours. Aside from keeping plenty of both RO/DI and mixed saltwater at all times in case you have to use it, having several sturdy containers that can hold all of your rock and fish along with a heater and pump/powerhead for each one can also be a good idea. If something bad happens, you might even use something like your bathtub to hold everything if you are desperate, but it’s better to be prepared.

Herbie Overflow Method Reef Tank Plumbing Guide


The method was popularized by a thread started by a forum member “Herbie” around 2004. As people started using the method on their tanks, many forum threads started and there are literally hundreds of posts out there about it. The difficulty lies not in the method itself, but in sifting through all those bits and pieces of info just to figure it out.

About this guide and a heads up

This guide explains the regular operation, each component of the design, and gives tips along the way. For the sake of simplicity, all of the content and diagrams describe the most simple and conventional Herbie setup, — an overflow box with standpipes coming in through the bottom. In some cases the pipes will have to pass through the side of the tank, but the mechanics of everything will be the same.

You should be aware that:

  • It is not completely a “set-it-and-forget-it” method… It needs to be regularly inspected and properly maintained. See Best Practices
  • There are all kinds of people out there who run the system in one form or another, and they all have a slightly different opinion on how they think things should be done. I am only one of these people. the most important thing is to understand your own setup. You can choose to do things differently with certain parts of the design, but try to be aware of any risks you may be opening yourself up to when you do so.
  • There is another similar method called the “Beananimal” drain (also named after its creator) which is slightly more complicated in design and uses 3 standpipes vs. only 2. The Beananimal is superior in many ways including running a “dry emergency” drain (increased safety), and having much more capacity than the Herbie design.
  • For the sake of clarity, I won’t attempt to explain the Herbie and BeanAnimal designs at the same time. The goal is that you will not only understand the Herbie design after reading this, but start to see it’s limitations.

The Siphon that makes the Herbie work

Any time you have air and water flowing through the same drain, you’ll get noise with even low amounts of flow. This is the main shortfall of single-pipe-drain methods like the Durso and it’s siblings. Aside from running silently, a siphon drain has much more flow capacity because air never runs through the pipes once it gets going.

The way the siphon on the Herbie is achieved is by manually constricting flow on the main drain with a finely-adjustable valve. Once the air is purged from the line and the siphon is fully engaged, it continues to run this way 24 hours per day until something interrupts flow like the return pump being shut off.

The rate that a siphon moves water is similar to a return system.

For a return pump, the greater the distance the pump has to push water up to the tank against gravity, the less flow exits the return outlet.

So for a siphon, it is working with gravity — water moves through the pipe faster the further it has to fall straight down.

So as you increase vertical distance between the inlet in the overflow and the outlet down in the sump, water moves faster.

Ideally the siphon pipe should run straight down to the sump, with no excessive horizontal runs (anything less than 24″ generally is no problem). This isn’t always possible, but you should keep it in mind. Flexible pipe can make curves more gradual, and can using 45° fittings instead of 90°’s for bends on rigid lengths of PVC.

Adjustment of the Valve

valve aquarium

To begin, the valve is opened all the way — then slowly closed off until the siphon is achieved. Fine adjustments are then made until the flow of the siphon closely matches the amount of water being returned to the tank, which will stabilize the water level in the overflow.

Perfectly balancing the water level until it remains totally still inside the overflow is difficult and time-consuming to achieve even with a Gate valve. Although it can appear stable and stay that way for a while, slight variations in the system such as changes in resistance of water through the plumbing will inherently change the rate of flow over hours or days. For example, the growth of slime in the pipes.


Simply by allowing a very slight trickle of water to enter through the emergency standpipe at all times, you sidestep the nearly impossible requirement of perfectly adjusting the valve. We are talking a trickle here — just enough to negate the need for constant fiddling, but still allowing the emergency drain ample capacity if needed.

To accomplish this, you simply stabilize the water level as best you can inside the overflow, then close off the valve’s flow a very tiny amount past that point. After this tiny adjustment, the water level inside the overflow will rise, very slowly, and come to rest at the level of the emergency standpipe — where the trickle of water will begin seeping down it.

So what is a Trickle?

Using a term like “trickle” leaves the actual amount to interpretation.

If you fill a styrofoam cup with water then poke a hole in it with the lead of a pencil, you will get an idea of what to aim for (and allow) with your trickle. Think “seeping” and not “flowing”.

The Effects of running a Trickle

A slight trickle of water flowing down the emergency pipe has a few effects on the system including:

  • Inside the overflow box, the operating water level remains up at the top. Water passing over the the weir falls only a short distance, so it won’t splash and create noise.
  • A trickle of water breaks the surface of water inside the overflow, preventing any scum from accumulating.
  • The need for constant adjustments is reduced considerably. Sounds great, right?
  • But don’t forget – Your Emergency line goes from dry to wet. Running any amount of flow down the emergency adds risk to the system. It’s a level of risk that many choose to live with and minimize the best they can, but present nonetheless.

Let’s Pause Here… For the sake of discussion, I’ll mention a debate that currently takes place. It regards running an Emergency drain with the Trickle method. There are some people who think that running a strictly Dry Emergency that never touches water is the only way it should be done.

I have the opinion that running a Trickle down the Emergency drain can be done in a safe manner in conjunction with following best practices like using a strainer on the siphon inlet, overflow teeth on the weir and a covered overflow box. I wanted this guide to be just about explaining the basics vs. debating different opinions, so that subject is in a separate post here Herbie Overflow Dry Emergency vs. Running a Trickle

Moving on…

System Design

Valve Selection

Due to the need for fine levels of adjustment, installing a quality valve is suggested.

Gate Valve

This valve allows precise adjustments to be made with a minimum effort. The “Spears” gate valve (shown) can be taken apart and is regarded as the best available option.

Gate Valve Herbie

Spears Gate Valve

Ball Valve

The higher quality ones have a smooth opening/closing action, and built in unions which are an advantage. A “Single Union Ball Valve” or “True Union Ball Valve” are the only ones that should be used. They are cheaper and easier to find than gate valves. The common “Straight Ball Valve” should be avoided.

True Union Ball Valve Herbie

True Union Ball Valve

Think Ahead – Add A Union

If the valve doesn’t have a union built in — one should be installed between the valve and the bulkhead so you can easily detach your hoses and get to the valve assembly with relative ease.

Valve Placement Vertically on Main Siphon Drain Line

The vertical position of the valve on the siphon line plays a part in how fast the siphon starts.

All things being equal, the siphon will purge air and start faster if the valve is down closer to the pipe’s outlet. This is because as water fills the pipe, the air exits the high section of pipe easier than the low section that’s after the valve. That is — less air needs to purge and exit the pipe downwards.

A simple and common way to run your setup is with the sump in the stand below the tank. This means the siphon line coming down from the display tank is only a few feet long. You can help the siphon start quickly by doing a few things: Plumb the drain with a minimal horizontal run, and have the drain outlet the proper height.

When the siphon is re-starting, such as when you restart the return pump, it will take up to a minute in some cases to get going again.

By placing the valve all the way down at the sump just before the water level, the pipe will go full-siphon the quickest. But in reality, it does not matter much whether the siphon goes into silent operation in 30 seconds vs. 45 seconds for example.

You can place the siphon valve directly below the bulkhead, or all the way down at the outlet. The difference in time to go full-siphon is only a few seconds with most tanks. So having the valve in a convenient spot to reach can often take precedence.

Dialing in the valve can often take a minute or two until you get the hang of it. So if you place your valve where it’s easy to reach, then you’ll thank yourself later.

On my tank (shown in pic above) the valve is just below the bulkhead. It’s a lot easier to dial in the flow when you have your hand on the valve and a line-of-sight to the water level in the overflow. In this case adjustments are made standing next to the tank rather than crouched in the cabinet.

In cases where there is a long run in the siphon line, such as having your sump in a basement, low placement is much more important. In this case the valve needs to be located it down near the sump to prevent trapped air in the pipe.

Main Siphon Standpipe Height

Herbie Overflow drain heights

A sufficient volume of water above the siphon inlet prevents air sucking down from the surface into the pipe, sometimes referred to as a Vortex. If one forms, it creates a slurping noise. They will also interfere with the line going into full-siphon.

6″ is a general rule. It is based on the rough dimensions of many overflow boxes.

Keep in mind though, it is not as much about how deep, but rather how much water volume is above the siphon inlet. More water volume above the inlet = more water forcing the air down the drain to create the siphon effect.

Many of the commercially available overflow boxes are made as small as possible. Why? They are easier to ship, use less material, and have better aesthetics.

But with a small overflow box, there is less water volume to purge air down the drain. So the 6″ might not be enough in those cases, or with a larger box, it may be more than you need. The height of the siphon inlet in the overflow is something you may want to experiment with.

If you forgo the raised main standpipe (such as on the right of pic above) it will take the guess work out of it, just be warned that ALL the water in the overflow will drain into the sump when you shut off your return pump. This water needs to have a place to go without overflowing the sump. Also – any livestock in there (even if they shouldn’t be) will be left high & dry.

Emergency Drain Setup

This drain is what makes the system fail-safe in the event of the main valve standpipe getting clogged. It should be designed simply as a straight (as possible) unrestricted pipe going down into the sump and into the water, never joined with other pipes.

An open-ended standpipe will act as an alarm in a way. With only a small trickle of water seeping into it, it will make no noise. If there is a partial blockage of the siphon line, then the Emergency drain will start to take on more water and you will hear a loud hollow noise, alerting you that it needs attention. This is one reason that you should not try to muffle the Emergency drain.

You want the emergency to have max capacity, so most people leave it open and don’t use a strainer on this pipe.

Don’t install a valve on the emergency pipe — just don’t…no good can come of it.

Emergency Drain Standpipe Height

As a measure to prevent noise from falling water in the overflow, the emergency standpipe inlet should only be around 1/2″ – 1″ below the height of the overflow weir.

You want the height of the E-drain inlet to minimize water’s falling distance over the weir… but just high enough to limit splashing, no more.

The distance between the E-drain inlet and the top edge of the tank needs to be a few inches to aid with capacity. This is needed in the event of a main siphon line blockage.

If this happens, the E-drain will possibly need to go full-siphon as well. If the standpipe places the inlet too high up, a siphon may not be able to engage on the E-drain if it has to before the tank’s water level swells to the top edge of the tank and starts to overflow it.

Starting and Stopping

When the return pump switches on, and water starts passing through the overflow, the siphon should engage and go back to normal operation completely on it’s own. While this is happening, you may observe a temporary increase in water going down the emergency standpipe. This should only go on for a minute or so until all the air inside the main siphon line can purge through the outlet down in the sump. The siphon will then engage and take on the full capacity you adjusted it for.

Under the tank

Drain line Outlets in the Sump Below

Both drain outlets should run down from the overflow into the sump below. The siphon drain must have it’s outlet submerged, but by <1″. Any deeper, and air can’t purge from the outlet as easily when the siphon is starting. It should also be located in a sump chamber with a constant water level.

The emergency line is usually also submerged to limit any sound from splashing from running a trickle. This outlet should also be submerged to a depth of by 1″ or less. If the main valve siphon gets blocked, the emergency pipe will take on the extra water. If the e-drain needs to go full-siphon, you don’t want to impede that by submerging the outlet down too far because that is where air is purged.

In the diagram, the emergency drain is shown right alongside the siphon drain. The e-drain does not need to go into a particular sump chamber. It should be plumbed the way that provides the straightest path down to the sump without horizontal runs.

Sump Capacity

When the return pump is shut off, the water level inside the overflow will drain and come to rest at the level of the siphon inlet. The height difference between the emergency inlet and the siphon inlet will determine how much water will drain to the sump. Make sure the sump can accommodate the extra water without overfilling.

More in-depth info about sumps and what to keep in mind with a siphon-based overflow method on this page: Reef Tank Sump Design

Avoiding Common Pitfalls

Trying to Tee the drain line

It is also important to note that you should not attempt to Tee off either drain, as it will most likely interfere with the siphon operation, capacity of the emergency drain or cause other problems.

Horizontal Runs of Pipe

Both the emergency drain and the main siphon drain should be run straight as reasonably possible downwards into the sump, as any excessive horizontal runs will make it more difficult for all the air inside the line to exit the pipe down in the sump as quickly. It’s generally only an issue if a horizontal run is over 24″ long. If you have sufficient height, you can get away with longer horizontal runs without too much issue.

For the Return line, horizontal runs are fine. This is because returns have water being actively pumped, rather than draining due to gravity only.

Best Practices for a Problem-Free Overflow Box

Doing these things will make the main siphon less likely to clog.

Use a Strainer

Install a Strainer on the Main Valve Standpipe. This will keep any debris, small snails or algae clumps from interfering with the valve and choking off the drain. There are different strainers out there, and one with smaller holes will protect better, but clog with junk faster. Try to get in the habit of inspecting the strainer when you observe and check on your tank. After all, you want it to prevent clogs, not be the source of one.

You might be thinking, “I don’t need no stupid strainer” — YES YOU DO!

Listen up.

With the nature of a siphon, water moves very quickly through the valve opening. Most times you’ll find that the valve needs to be closed-off in excess of 50% shut to achieve balance from the return. A mostly closed-off valve opening is very small during operation. This makes it more susceptible to obstruction by an object.

Having a strainer in place is the last line of defense before your main drain clogs.

Cover Your Overflow Box

A covered overflow will prevent livestock in the tank from jumping or crawling inside. This is a good idea to further protect the valve against clogs, and can also help to limit algae growth. You’ll also save yourself having to catch a stray fish inside the overflow which is a real pain. A simple piece of acrylic cut to size does the job.

Visual Inspection of the System

You might want to place your overflow in such a way that you can easily look into the box. You want to know:

  1. Is the water level where it should be?
  2. What is the condition of the strainer? Does it need cleaned?
  3. Is there any debris or livestock in the box?

Other Considerations in your System

Is my tank’s glass even suitable for drilling holes for bulkheads?

Drilling and mounting bulkheads to a tank adds lateral weight and forces on the glass. What this means is that the leverage from plumbing pipe and fittings will create pressure points on the area around the glass hole.

These physical forces determine which aquariums are even suitable for drilling. Glass with thickness of at least ¼” can typically withstand these forces, and quarter inch (6mm) glass is generally only found on aquariums with capacity of 40 gallons and up.

Don’t waste your time or risk having a future catastrophe later on by drilling a tank that isn’t suitable. If the glass is too thin and weak to support bulkheads, then it’s a lot more likely for cracking to occur.

As always, check to make sure the glass you are drilling is not tempered. There are many videos on drilling glass on YouTube and it’s easy to do with diamond hole saw bit. Drilling techniques and hole placement is covered in the Drilling Holes in Glass Aquarium Tanks Page

Size of plumbing

There is a general consensus that the Herbie method should never use less than 1″ pipe for plumbing, no matter what size tank. You want to be 100% sure that your single emergency drain is completely capable of handling all the flow that the return pump can throw at it in the unlikely event of total clogging of the main drain.

If your tank came pre-drilled with 2 holes in the overflow as in a lot of older “reef-ready” tanks, use the one with a larger diameter (usually 1″) for your emergency and the smaller diameter one for the main siphon drain. Keep in mind that your flow will be drastically reduced on smaller size pipe. – (See Figure 5 for picture).

For systems where return pump flow is up to around 330** gallons an hour, running a 1″ emergency line will be sufficient. For tanks above about 120 gallons you may want to bump-up your plumbing size to 1.5” in diameter. These can handle way more flow.

The trend in the last few years is to use efficient powerheads to create high flow in the tank, while running a moderate amount of flow through the sump. This means you can run a smaller, quieter return pump and not push the capabilities of your overflow design as much which is a smart strategy.

Return pump plumbing

The return pump also plays a part in the operation of the overflow and in short, simple is better. Any variations you add into the plumbing of the return line can have effects down the line.

To avoid having to constantly readjust the siphon valve, the return rate should be kept at a consistent rate. Any fluctuations that would take place from day-to-day should be minimized.

A common practice is to to Tee off the line above the pump to feed a Refugium sump chamber, and this is generally okay to do.

If you are going to Tee the return line, the offshoot should only direct to a place providing consistent resistance.

Some people will add a manifold of valves to direct flow to media reactors. This can be less than ideal with a siphon-based overflow method.

If the return pump is Tee’d off and also feeding a media reactor in the sump, then variations in water flow will start to happen. Water resistance inside the reactor will slowly change as the media becomes depleted or dirty. This in turn changes the rate of flow pushed up to the display, then falling over the weir and into the overflow.

All this means is that if the setup allows for variations in flow, the valve on the siphon will need to be adjusted more frequently.

More in depth info about plumbing return lines in this article here: Reef Aquarium Return Line Plumbing – A How-to Guide

Assembly and Gluing

Inside the overflow, your plumbing fittings can generally be slipped & threaded together. These fittings don’t need to be glued as there is no risk of leaking externally. Having your overflow box fittings not glued allows you to make changes while initially testing out the system. Trying various pipe lengths for the standpipes inside the overflow is often needed.

On the exterior of the tank, fittings and pipe should absolutely be glued with PVC solvent (available at any hardware store). Installing unions between the outside of the bulkheads and the plumbing lines/valve is a good idea and allows for the hoses to be removed for cleaning or if you have to move the tank.

More info about how to glue and connect the various pieces: Plumbing Assembly for your Reef tank

Testing the system

There are a few initial tests you should do with the system to ensure problem free operation for years to come. Any problems are better to find out in the garage than in the living room. As long as it passes these tests, then you are ready to go.

  • Close off the siphon valve completely in order to test your emergency drain’s capacity. You must confirm it can handle what your return pump is pumping up to the tank with ease. If using a controllable DC pump, make sure the drain can handle flow at 100 percent power.
  • Ensure you have the right sump capacity by turning off the return pump, then watch both the drain and return lines empty into the sump. Make sure it can handle the extra water (also watch your skimmer).

What about Freshwater?

There is no reason why this method can’t also work for Freshwater tanks. One thing to note – if you have Freshwater plants, there will be a lot of dead pieces/clippings constantly getting into the overflow system. These can clog overflow teeth, or a strainer. Use care.

One Final Note

Ultimately you are responsible for your own tank and your own property — there is the potential for water damage that comes from owning any aquarium. As I have said earlier, the most important thing is to understand how your own system is set up. Learn as much as you can and don’t cut corners. Enjoy your quiet tank!

About this Page

While my original post was written in June 2012, it has grown into a full guide. I have made several edits to clarify some of the concepts and added a few sections based on some if the comments left below and various forum threads linking here. Thanks to those who have participated in the discussion. Ask me a Question using the Contact Form or write it in the comments below.