Reef Aquarium Sump Tank Design

Reef Aquarium Sumps are something that can be very simple and easy to set up if you know a few things about what you are trying to do.

A big reason why sumps are confusing to beginners is that a lot of people make them way more complicated than they need to be. They see all kinds of pipes going all over the place and think they must be hard to build.

Depending on the stand or cabinet you are using, it can be a hassle to remove or change your sump setup. It is worth putting some thought into how you are going to use it and what it needs to do.

Sumps are simple, and you can turn a $20 Craigslist tank into one that functions just as well as $300 commercial models. This article will go over what I think is the simplest method to set up a sump in a bare tank, with all the reasons behind the design.

What are the main benefits of running a sump?

  • act as a buffer-zone where water moves to be filtered mechanically and chemically
  • slowly process water in such a way that promotes stability of the water and avoids spikes in water chemistry and temperature
  • let you run equipment out of view, so you can have a clutter-free display
  • contain any leaks from equipment or reactors from reaching the cabinet or floor

What we’re trying to do here

  • catch the water from the overflow drain outlet
  • deliver water to the equipment that processes it
  • remove any microbubbles that may have been generated by the equipment
  • return the water to the aquarium as quietly as possible

But we also want to make sure we:

  • have enough room to deal with back-siphoned water when the return pump turns off
  • have fail-safes in place where possible to prevent floods
  • cooperate with the Herbie, Bean or “siphon-based” overflow designs

reef tank sump design

Size Considerations

Most of the time, you’ll want to have as big of a sump as you can fit in the area. While you may have heard about “added stability” with a larger water volume, this is usually not a noticeable thing.

Having a larger sump just makes a few things easier and less complicated. A bit of extra room to work with means being able to run just about any equipment you want. It’s nice to have some space available for extra live rock, reactors, and those sort of things.

The 3-chamber design shown in this article will work for the majority of larger sumps. If a small tank must be used – say 10-20 gallons – then you will probably have to alter the design and not use the center chamber. With a small sump, a 2-chamber design will allow the return pump chamber to hold enough water.

How much turnover should we have?

There are not many benefits to running high rates of turnover through your sump, other than having more water pass through mechanical filtration. Each individual piece of equipment processes water at it’s own rate, but most can process a very small amount at a time. For that reason I generally recommend a rate around 5X-7X of the display volume per hour. More on turnover rate here: What is the right turnover rate for a reef tank?

1st Stage – Where water from the overflow enters the sump

The first chamber will have the highest water level and then cascade down to the other ones. For the first divider, you want the water to pass over a weir of some kind.

The reason for having water go over a wier is to maintain a consistent water level in the first chamber.

A consistent water level is something we need for the skimmer to work problem-free, and it’s also helpful if you have a siphon-based overflow. In order for the siphon to start, the outlet of the pipe must be submerged, but only by about 1″ (3cm) below the water surface. A siphon can’t start correctly otherwise.

Designing for your Skimmer

Most new skimmers available are non-recirculating models that operate in-sump. They usually have a recommended water height from the manufacturer they are designed to work at. This usually at a point between 7-12 inches…

When a skimmer is dialed-in and properly adjusted, it’s pretty touchy. If the surrounding water level in the chamber rises, it reacts by overflowing the collection cup and skimmate collector. Not fun to clean up.

If you design this chamber to have all the water flow to the next by passing over a weir, this won’t happen.

So if you are trying to figure out how tall to make the divider between this first chamber and the next, you may want to choose something close to 12″. You can boost a skimmer up to the right height with a DIY stand of some kind, but you can’t lower it if it’s already too high…

Having ample water depth in the first chamber gives you options. It allows you to change or upgrade your skimmer, just by adjusting the height of the platform.

Dealing with Back-Siphoning

Water in the pipes and in the overflow will go in the direction of gravity when the return pump is off. This can add up to several gallons of water that flows down to the sump.

There are ways to reduce this amount of water by correctly designing your Return Line and Overflow System, but no matter what there will always be some back-siphoning of water.

This is why it’s nice to have a bit of elbow room when it comes to your sump’s capacity.

Ways to Minimize Drainage

Before we even get to planning out the layout of the sump, it is beneficial to reduce the amount of water that back-siphons and drains into it.

This is done by:

  • Return line outlet only submerged about 1/2″ below the surface of the display tank.
  • Correct height of standpipes in overflow box – water will drain to level of lowest inlet in the box when the return pump shuts off.

Stage 2 – Housing Equipment (Optional)

The middle chamber of the design can have many uses. This can be an area for reactors, heaters, or anything else. Hanging lighting over the area for a frag rack or refugium area is also common. You will most likely want a constant water level in this chamber too. This avoids the risk of having pumps or heaters running dry from a low water level.

Return Pump Chamber – Where the water level fluctuates

Your return pump goes in the last stage of your sump, how ever many chambers that ends up being. The water level here will lower as water evaporates, and raise when you top off the system with fresh water. So some people will mark the glass to show what the water level is supposed to be at when the system is topped off.

A piece of masking tape to indicate the high water mark can be helpful. Make sure that when you mark this point, the return pump has been shut off and all water has collected in the sump from the overflow and return line… That is the true high point.

Making sure the sump doesn’t overflow

By designing the sump to have water flow over a series of weirs, we control where the the water level will change. As it falls from chamber to chamber, it will collect at the end. This last chamber’s water level will vary based on evaporation or top-offs.

So this means all the other chambers up the river (the ones where water passes over a weir) will remain consistent.

A cause of frustration among first-time sump builders is overfilling. It is often not discovered until the plumbing is done and the return pump is turned on, then turned off.

It can be tempting to make your return pump chamber very small, because that is probably the only piece of equipment that is going in there. Might as well make lots of room for the other chambers, right?

This can have unintended consequences, which I’ll explain.

When the return pump shuts off, gravity takes over. Water will start to drain from the overflow box, display tank, and inside the plumbing. All this water collects in the return pump chamber. If it can’t handle the volume, the water will fill the other chambers up river. This can result in the skimmer overflowing or in the worst case the whole sump.

Overfilling when the return pump is shut off is the result of a simple problem — The chamber where it resides is too small.

Contrary to many sumps you have seen, the return pump chamber will often be the largest chamber in the sump.

The good news is that overfilling due to back-siphoning is easy to avoid. By calculating the return pump chamber volume beforehand, we can estimate how big it needs to be in the planning stage.

Determining Return Pump Chamber Size

First, we know that the return pump rests on the bottom of the sump.

It’s meant to be fully submerged, so we need at least the height of the pump itself as a minimum water level.

As the water evaporates from the system, the water level falls in this chamber. So at a certain point, the water level will get low enough to affect the inlet of the pump. That is – air will be drawn in through a venturi that forms.

If this happens, a slurping sound will be heard.

The typical pump won’t slurp air unless the water level is right above it. Having the water level 2-3″ above it is plenty of room to keep this from happening.

This will be about the “minimum” water height of the return pump chamber. This level will also be the normal operating level — while the return pump is on.

For example, in this case we will say that 6″ of water is the minimum water level that a return pump can operate without slurping or having a venturi form. Above the 6″ mark will be the base of our needed “extra capacity.”

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Even with proper design, there will always be a few gallons of water that drains down from these sources:

  1. Overflow box (drains to lowest inlet or standpipe)
  2. Display tank – the amount of water between the height of bottom of overflow weir and the height of the lowest point of the return outlet (usually only about 1/2″ of depth if you design it right)
  3. Inside the pipes (not very much unless using long runs)

So by adding these three sources together, we calculate the “extra capacity” needed when the pump shuts off.

When the extra capacity is added to the minimum, we find out the total required capacity of the return pump chamber.


Even if you decide to forgo these calculations and just use a check valve, there is another issue with creating a return pump chamber that’s too tiny. There will be a lot of swing in the water level height. This means that the water level in that chamber will rapidly go down as water evaporates from the system, and rapidly rise when the return pump is shut off and water empties from the pipes…

So why does this matter?

Well if you are using an ATO (and you should for many reasons), then you will want it to top up the system a few times per day, but no more than that. If the chamber’s volume is too small, then the ATO’s sensor will trip constantly — possibly shortening the unit or pump’s life from overuse.

For this reason you might want to make your return pump chamber larger, just to avoid this excessive swing in water level height.

Teeing Off the flow of the Return

Some people will use their return pump to also feed the refugium or one or more reactors. This means they they Tee off the return line and direct some of the flow over to that equipment.

A downside to Teeing off the return to feed either a skimmer or a reactor is that it will affect the return rate up to the display.

The return rate affects the overflow. If you have a siphon-based overflow like a Herbie or Bean Overflow, then variations in flow will mean re-adjusting the valve. This can be annoying to have to do all the time.

These variations in small amounts of flow is one reason why the Bean Overflow design has an advantage over a Herbie design. The Bean has the ability to take on variances in the rate of flow through it’s open channel, where the Herbie would need to have the valve adjusted.

So if you’ve got a siphon-based overflow, the system works best if the return pump’s only job is to push water up to the display. The flow may be Tee’d off to provide flow to something that provides consistent pressure such as a refugium chamber, however diverting flow to a manifold to power media reactors generally doesn’t work well with a siphon based drain. This is personal preference because most people don’t want to deal with frequent adjustments.

Sump Construction

caulking gun

The dividers of a sump are called Baffles — vertical panels siliconed in place. The configuration of the baffles determines how the water flows from chamber to chamber.

In a glass sump, you should use glass baffles. Using silicone to affix acrylic panels to glass doesn’t work well in general as it will never create the same strong permanent bond as silicone to glass.

Dealing with Microbubbles

Tiny bubbles in the water are called “Microbubbles” and will blow around in the current of the water column, until eventually bursting at the surface. While they are harmless, they can detract from the crystal-clear look of the water. Common sources can be skimmers and overflow pipes, or anywhere water falls more than a few inches.

Bubble Traps are a series of 3 baffles placed one after another to ensure all the tiny bubbles dissipate. They work by moving the water vertically up and over a barrier where water is calm.

While they work well, they can take up a lot of room in the sump. If you don’t have a lot of room to work with, you might want to skip using a bubble trap. Most newer skimmers don’t produce many microbubbles at all, especially once broken in. Placing the skimmer near the drain line outlets on the opposite end of the return pump should create enough distance in order the for bubbles to pop before they make their way to the display. While not needed in every circumstance, it’s nice to have a bubble trap in place just in case you need it one day.

Building a Bubble Trap


Bubble traps are usually placed at the last divider before the return pump chamber. This way bubbles dissipate before getting pumped back up to the display tank.

Distance Between Baffles

As a general rule, the baffles in a bubble trap should never be closer than 1″ (3cm) to one another.

If you are passing water underneath a baffle, the panel should be raised off the glass the same distance. The height of the opening should be at least equal to the gap between individual baffles. This will avoid a bottleneck in flow.

While it may be the norm to only have 1 inch of space between baffles, it is really the bare minimum.

A gap of only 1" creates an area that is hard to access. A larger opening will be easier to clean between the panels. Furthermore, if you keep livestock in the sump, it can be a real pain to net something out from between 2 baffles if they are only 1" apart.

An opening of 2" is often better, not to mention easier to install in the construction phase. By creating more space between the baffles, you promote stillness in the water. Stillness helps bubbles rise to the surface.

Under or Over?

The diagrams on this page show under-over-under. Designing a bubble trap with this pattern is less common. There is no wrong way, but I find the under-over-under way has a few benefits.

If you make the bubble trap the opposite way – as over-under-over, it creates an area that is hard to access.

Over time, some detritus will settle at the bottom of an over-under-over bubble trap. Not a big deal to most people, but if you want to clean out that part, it can be difficult. The area at the bottom will be especially hard to access if the baffles are close together.

Also – if you keep fish in your sump from time to time as I sometimes do – over-under-over can be a pitfall. If a fish gets in that kind of bubble trap – you will have a hell of a time getting it out.

So under-over-under might be the best way, but it’s personal preference.

Either way of setting up a bubble trap will keep microbubbles away from the return pump. Ultimately, a slow rate of flow is the biggest factor in how well they work.

Installing Baffles in a Tank

There are many steps involved in selecting and preparing your glass baffles. This info has been split into it’s own post: Reef Tank Sumps: DIY and Construction

Things like measuring, cutting, glass thickness and prep work are shown. There are photos of a 40 breeder sump build with some helpful DIY techniques for working with glass and assembling the sump.

Refugiums and Sump Setups

Some people will install a refugium into their sump. While common, there are plenty of people that just do without them. The term itself has become a little generic. A “Fuge” often just refers to an area of the sump where there is lighting and stuff grows.

What is the Sump actually doing?

A repeated statement in reefkeeping information is that your refugium needs slow flow running through it. This is not true in all cases.

Some people will Tee off the return pump and divert a portion of the returned water to a “slow flow” chamber. Reducing flow like this is usually counter-productive if the refugium’s main job is to grow macroalgae.

Macroalgae grows the fastest with two things present. Good water flow (more oxygen) and lots of light in the proper spectrum. Reduced flow will just impede it’s growth.

So the question is, why bother Teeing the return line?

It is simple to just place the macroalgae in a sump chamber with consistent water height. Usually the macroalgae needs to be segregated or prevented from getting sucked into pumps. A plastic mesh divider or basket can be used in these cases.

Some refugiums are created as a habitat for microfauna and all the critters that live in the reef. These area are a true “refuge” and will usually have some kind of sandbed. Providing a safe place away from fish and predators will theoretically let them flourish and multiply. The ideal amount of flow for these types of refugium zones is a matter of debate and many people have tried to experiment with them but there isn’t really a consensus.

If you are building your first sump then you will probably get the most mileage out of trying your hand at growing macroalgae like Chaeto. A more complicated refugium setup can then be something you choose to experiment with or forgo altogether.

More advanced designs

In the diagram at the top of this article, I show water moving from one end of the tank to the other. The water cascades, flowing from chamber to chamber like a waterfall. Illustrating the concept this way is simply done to make the concept easy to explain and show.

This design is generally a good practice. When the dividers span the whole width of the tank (front to back), the water passes over a longer weir. Having water pass over a longer weir allows for slower flow that is quieter.

Sometimes you want to go a little further with your design. In order to maximize what the sump can do, having each divider span the full width of the tank isn’t always the answer. More creative and intricate sump designs are possible. Rather than just having rectangular pieces of glass, you can create openings for water to pass through.

Cutting notched panels with the corner cut out allows for the sump to do a lot more with the minimum amount of dividers. For an example of this check out the Reef Tank Sump DIY Construction post.

Containing Spills and Being Proactive

Having a large sump with fairly high walls allows you to have a bit more piece of mind.

Equipment such as reactors have the potential to come apart and leak. The pump that drives them will continue to go, and can put water where you don’t want it.

Reactors may also have flimsy barbed fittings and tubing, especially on the cheaper models. If one of these fittings blows out, you want the reactor in a place where the water will only spill into the sump and not anywhere else.

The same also goes for things like a Skimmer’s Skimmate Container. While common to be be located next to the sump, what is to happen if it overfills?

Anything with the potential to leak should ideally be located inside the walls of a sump or just above it. It can also be a good idea to install a water alarm in the cabinet.

There are many ways that a tank can potentially leak, which you should always be aware of. For this reason I have also written this post here: Common Causes of Flooding on Reef Tanks