What is the ideal Flow Rate of Turnover through the Sump?

Turnover rate through the sump is something that people often overshoot while designing their system. They often end up with a louder pump than needed.

It helps to know a bit about how a sump works and how equipment is affected by the turnover rate. This will help you to make the best decision about how much water needs to pass through your system.

The Purpose of a Sump

Many people coming into the hobby do so from the freshwater side of things. Take for example a freshwater cichlid tank with many fish. When you are dealing with a lot of fish and trying to keep the water clean, mechanical filtration does most of the work.  Moving a lot of water through the filtration media is better in that case.

Mechanical filtration is only one part of a typical reef sump, and is sometimes left out completely. Most of the equipment present is not affected by the turnover rate.

A sump on a reef tank acts more as a buffer-zone where water can move to a separate area, to be filtered and chemically processed. But it isn’t a matter of dirty water going in and clean water going back up to the tank. In reality, a skimmer or chemical filtration reactor will only process a fraction of the water that passes by it. The majority just moves right on past. So rushing a ton of water through the sump provides no benefit for the skimmer or reactors to work any better.

What is the optimal Turnover Rate?

The 10X rule and the reasoning behind it

One thing you still hear quite frequently is that you need 10X or more times your system’s volume every hour when choosing a return pump. (An example would be a 90G system = 900GPH).

While the 10X rule is a nice round number, a turnover rate of 5X-7X of system volume per hour is often more than enough.

The 10X number has been around a long time. It’s hard not to think that most of the people who came up with it were basing it on the equipment that was available a few years ago. Older impeller-style powerheads were much weaker and less efficient than the ones we have today. Many of these newer pumps now have wide flow, and wavemaking capabilities.

Using some of these new propeller-style pumps means you don’t have to rely on the return pump to create random flow in the tank.

With most of the flow coming from powerheads, you don’t have to do anything fancy with the return line. Doing things like splitting the return outlet between multiple outlets generally isn’t necessary and can actually result in less flow than a single line. Same goes for using loc-line fittings to point the jet inside the tank. All this fiddling and over-plumbing really does is restrict the return pump output.

If you are trying to select a pump that is quiet and well suited to the job, just use a single outlet. Large enough pipe will also help to maximize the output of your pump.

Why you might want less turnover

By having a lower rate of turnover through the sump, there are a few advantages like:

  • A smaller return pump can be used, which has a few advantages of it’s own. A smaller pump costs less to buy, and saves money in the long run by using less power. Not a bad idea for a device that runs 100% of the time. There is also a pretty big difference in both heat and noise between a 500GPH pump and a 900GPH one.
  • You can stay well within the capacity of the overflow. A lower rate of return will also skim off a thinner layer of water passing over the weir. This decreases the chance of fish or debris going through the overflow teeth and causing problems at the inlets / standpipes
  • When adding chemical additives or top-off water to the system, they will enter the display at a slower rate. This ensures changes to water chemistry don’t happen too fast. The theory is this is easier on the livestock. We’ll have to ask them if it makes much difference.

Surface Skimming, Agitation and how to decide on a turnover rate

The surface tension qualities of water mean that organics in the water are naturally drawn up to the top 1mm or so. This is one reason why it is so beneficial to drain water from an aquarium via an overflow. By allowing the surface to renew, you are helping to add oxygen to the water and remove waste through a natural process.

So having a good rate of turnover (more GPH through the return pump) will mean more water goes over the weir of the overflow every hour. So more stuff will make its way to the sump and skimmer. This “stuff” can be dissolved organics and solid particles in the water.

But how are you to know what "adequate surface skimming" really means? Well – it depends.

This is yet another area of reefkeeping that is up for contention. Most would agree that surface skimming is important, but it is hard to quantify. The difference in results between a coast-to-coast overflow and a conventional one are mostly anecdotal.

It would seem that surface renewal is important to an extent, but it’s importance is often overstated. Many overflows can provide decent surface skimming, provided the overflow teeth are designed right. Sometimes this can mean cutting out the existing teeth and replacing them.

The bottom line is you will find examples of successful tanks that have all types of overflows imaginable. Most of them can attribute success to good tank husbandry, not what the turnover rate is or how wide the overflow weir might be.

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.