Aquarium Sump

Having an aquarium sump as part of your saltwater system offers several advantages:

  • A sump provides a place for us to place our protein skimmer, chemical media like carbon and Poly-Filter, a phosphate reactor, heater, and the return pump for the chiller.  An auto-top off system if utilized, is best incorporated into a sump.  Electronic probes for measuring parameters like pH and temperature can also be placed neatly in the aquarium sump in custom built holders out of sight of the main display.
  • Dosing additives into the sump rather than into the main display is less stressful for delicate corals.  Dosing this way gives the additives time to mix well with the aquarium water before the return to the main display tank.
  • An aquarium sump increases the overall water volume of the system.  As we learned in our page on Why Beginning Hobbyists Should Avoid Starting A Nano Aquarium, the bigger the water volume, the more stable the water parameters in an aquarium will be.  And you’d be surprised how much more water an aquarium sump can add to a system — a 75-gallon aquarium with a 30-gallon sump would have a total water volume of 105 gallons!
  • An aquarium sump provides better oxygenation for the system.  By virtue of having an overflow and a sump, water from the aquarium is agitated and aerated as it splashes into the sump.
  • Some aquarists claim to experience a drop in temperature by as much 2 degrees F when a sump is utilized.  This has not been my experience as protein skimmer pumps and return pumps impart some heat to the water.

Aquarium Sump Design

The interior of an aquarium sump is usually divided into three or more compartments — usually glass plates that are siliconed in place to act as dividers.   The larger the sump, the more creative the aquarist can be with the different sections of the sump.

aquarium sump

Aquarium sump with protein skimmer and filter media

The first compartment receives the water from the overflow of the the main display tank.  This ‘raw water’ then flows into a second compartment where the in-sump protein skimmer is normally placed.   It is common to have a low partition below the water line installed in the first compartment, dividing it into two halves  — one half to receive the raw water and the other to place the in-sump protein skimmer.

In-sump skimmers do not perform at their best when there is water turbulence, especially when placed in the same compartment as the roiling water returning from the display tank. Dividing the first compartment in the aquarium sump creates a ‘skimmer chamber’ to help minimize turbulence from the returning water.  The low partition ensures that water is free to reach the skimmer chamber.

When a gravity-fed out-of-sump skimmer is used, the return plumbing from the overflow is split in two — one side to provide a gravity feed of raw water to the skimmer and the other to enter the refugium section of the sump so that nitrates can be processed by the macroalgae.  Alternatively, the first compartment can contain a pump from which the out-of-sump protein skimmer will receive the raw water.

It is important to have the raw water from the main tank processed directly by the protein skimmer before it reaches chemical media such as carbon, Poly-Filter and phosphate remover.   Placing a skimmer in the last compartment of the sump, for example, will skim away valuable microfauna and organisms like copepods which are natural food for fish and coral.

aquarium sump skimmer compartment

Aquarium sump skimmer compartment

The first compartment of the aquarium sump is also where I prefer to place the pump for the chiller.

If one is creative with design, the second compartment can hold media trays for carbon, Poly-Filter, Purigen and bags of passively run GFO for phosphate removal.   The second compartment is also a good location for a phosphate reactor.

The third compartment of the aquarium sump is where I like to establish a refugium with live rock and macroalgae such as chaetomorpha.  Is is also a good place for bags of Seachem’s DeNitrate or Matrix for nitrate removal.  The refugium compartment is also where some aquarists like to place live sand or even a deep sandbed, which I personally find unnecessary.   And since deep sandbeds are rendered useless unless fine sand is used, there is a potential for fine sand grains to find their way into the return pump, ruining the impeller.

If using a chiller, the returning chilled water can be directed into the refugium chamber, providing good circulation and water movement around the macroalgae and live rock.

Aquarium Sump Sizing And Water Volume

Exactly how big should your aquarium sump be?

Typically, the sump tank should be about 30 to 40% the size of the main display.  Smaller volumes are certainly possible as long as the sump is large enough to retain the water from the overflow chamber and plumbing in the event of a power outage as we will see later.

The operating water volume in the aquarium sump is never allowed to rise too high.  With the return pump running, keep the operating water level in the sump a good 8 inches below the top edge of the sump tank.  This is your safety buffer to prevent the sump from overflowing, as we will discuss below.

To prevent an aquarium sump from overflowing in the event of a power outage, it is important to have a large enough sump tank that will be able to hold all the water as it flows back from the overflow chamber and the return plumbing.  Unless your sump is too small, or if the operating water level in the sump is too high to begin with, the water from the overflow chamber and plumbing will cause the water in your sump to rise and eventually spill over the sides and onto the floor.

Turn off your return pump and check the level that the water rises to and where it stops.  If the water is, at most, 3 or 4 inches from the top of the sump tank, you can be assured that the sump will not overflow should your electricity shut down or return pump fail.

Another thing to watch out for is to prevent the return nozzles below the water line in main display from creating a siphon which will also overflow the sump.  Drilling two 1/4″ holes in the sides of the return nozzles slightly under the operating water line will break the siphon effect.  With the return pump off, once the water in the display begins to drain into the sump, the water level will lower and the 1/4″ holes you drilled will suck in air and break the siphon.  The siphon break is characterized by a loud gurgling sound, followed by..  silence.  Ensure that these siphon breakers are never clogged with algae!

Alternatives To A Conventional Glass Aquarium Sump

Almost any square or rectangular plastic container of decent size can be customized to function as a sump.  Some aquarists have been using Rubbermaid food-grade containers as a sump for protein skimmers, heaters, electronic probes and refugiums.  All one has to do is silicon in some glass or acrylic partitions to create sump sections.

For even greater water volume and when space is not a problem, oversized square plastic tubs can be used as aquarium sumps.   These are usually not installed in the cabinet below the main display tank but rather behind it in a ‘fish room’ or even in the basement directly below where the display tank stands.

Can I Run A Saltwater Aquarium Without A Sump?

Definitely. You do not need a sump to run a saltwater aquarium.

While having a sump offers many advantages, there are some saltwater aquarium hobbyists who go sump-less for various reasons — one of the most common being that they purchased an aquarium that was not pre-drilled with an overflow chamber.  Some also do not want the additional expense of running an 80 to 100 watt return pump 24/7.

But unless you do not mind your main display tank looking like a laboratory experiment with an assortment of pumps, tubes and hang-on-back equipment, a sump is still the way to go.

Read more about saltwater aquarium setups:

Aquarium Tank Size | Length, Width And Height

The Ideal Location For Your Saltwater Aquarium

Ready-Made Aquariums

Drilled Overflows Vs. Siphon Overflows

Aquarium Stands

How To Setup A Nano Tank

Aquarium Chillers



Aquarium Covers | Put A Lid On It!

Cooling Fans


What You Don’t Need For Your Saltwater Aquarium!

Aquarium Sump As A Second Display Tank?


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