If you’re new to reef aquariums, you may not be familiar with having a sump and are wondering just exactly what it is. We’re going to tell you all about it here.

Basically, a sump is like another aquarium that is plumbed to the main aquarium, often called the display tank (DT). We have some very good information about them on our forum. Sumps are a perennial topic because they can be as simple or as sophisticated as the aquarist desires.

Why do people use a sump with a saltwater aquarium?

There are lots of reasons.

1. Increase in water volume. The addition of a sump increases your total water volume. Since reef aquarium water chemistry is fragile—and it’s critical that the chemistry is excellent for the health of its inhabitants—larger water volumes are inherently more stable than smaller water volumes. For example, if you have some big fish creating waste (poop) in 50 gallons, the waste is greatly diluted if you also have a sump of 50 gallons because then you have a total water volume of 100 gallons in your "system". With the larger water volume you have more time to address, process, and remove the pollution in order to keep your water clean and healthy.

2. Convenience. Keeping a reef aquarium typically requires more equipment than having a freshwater aquarium. On a saltwater tank, you probably have one or more heaters, some kind of filter, and perhaps a skimmer. Some people have a UV filter, automatic top-off, and dose different additives. There is plenty of other equipment that can be added depending on the tank needs and style of the aquarist. Take note, however, that lots of people successfully run a very low-tech tank. The point here is that if you have a sump, then all or almost all of the equipment can be in the sump unseen, so it doesn’t distract from the beauty of your display tank.

3. Noise. The equipment mentioned in #2 can be noisy. If you have a sump under your tank in a cabinet or even in another room behind your tank, the display tank can be silent.

4. Safety. Dosing chemicals in the sump allows whatever you’re adding to spread and get diluted before it reaches the display tank.

What does a sump look like?

The sump can be as simple as a small plain glass aquarium or they can be very large and custom-made. Sometimes a sump is divided into sections for different things. For example, there might be a section just for the skimmer to sit. One section might be for a refugium—a large topic to cover in another article. Sometimes there are baffles that make the water flowing through the sump travel over and under dividers which slows it down and helps to cut down on microbubbles. Marc Levenson of melevsreef is famous for his collection of articles about sumps if you would like to learn more.

Here is a sophisticated sump for a large system:


This is a royalty-free image from Pixabay.

Who needs a sump?

In general terms, smaller aquariums don’t bother with a sump (even though smaller aquariums would likely benefit the most). With smaller reef aquariums, it’s easy to do lots of water changes, which means you can probably get by with less equipment.

No one has to have a sump, but those with larger aquariums like over 40 gallons usually start to consider having one. A sump doesn’t have to look gorgeous, because no one will see it but you. Some aquarists even make their own sumps out of acrylic.

It will be up to you to decide if or when you need a sump. When you’re ready, we can help you on the forum to get it set up.


We encourage all our readers to join the reef2reef forum. It’s easy to register, free, and reefkeeping is much easier and more fun in a community of fellow aquarists. We pride ourselves on a warm and family-friendly forum where everyone is welcome. You will also find lots of contests and giveaways with our sponsors.

About the Author:

Cynthia White


Cynthia received her BA in English from NYU in 1492. She has been a freelance writer and editor for over 20 years. She has written articles for newspapers and magazines, both in print and online, as well as ebooks, press releases, and sales and marketing copy. She was formerly a Marketing Manager for a small oil company. Her portfolio can be found here. Now she is a writer and editor on staff at R2R, where her forum nickname is Seawitch. Her build thread can be found here.

For 15 years, she kept a dozen freshwater tanks, bred cichlids--Cyphotilapia frontosa--and sold them to pet stores in Calgary. Finally, after years of study and hand-wringing, she has come to saltwater side. She lives in British Columbia, Canada, with her husband and three special-needs dogs.