How to Set Up an Aquarium Sump

A sump is a second tank below the main one that holds the filter media and equipment out of sight. It is powerful, but not for every tank.

A sump is an extra tank — usually below the display tank — that water flows into and back from. It hides equipment, adds water volume and holds a lot of filter media. It suits larger tanks; small tanks rarely need one.

Why use a sump?

Hides equipment — heater, media and skimmer out of the display.
Adds water volume, which stabilises the system.
Big media capacity for biological filtration.
Easy maintenance — clean the sump, not the tank.

How it works

Overflow — water leaves the display tank into the sump (via a drilled tank or an overflow box).
Mechanical section — filter socks or sponges catch debris.
Biological section — ceramic or bio media house the bacteria.
Return pump — pushes water back up to the tank.

What to watch for

Flood risk — if the return pump stops, the sump must have room for the water that drains back. Design for a power cut.
Flow balance — the overflow and return must match, or the pump runs dry or the tank overflows.
Evaporation — a sump loses water fast; top up with RO to avoid salinity/pH swings.

For most beginner tanks, a good hang-on or canister filter is simpler and safer. A sump is worth it once you keep a large or reef tank and want maximum filtration out of sight. See choosing a filter.

Why use a sump

A sump does four useful things. It hides equipment — heater, media and skimmer — out of the display. It adds water volume, which makes the whole system more stable. It gives you a large media capacity for biological filtration. And it makes maintenance easier, because you clean the sump rather than reaching into the display. For a large freshwater or reef tank, that combination is hard to beat.

How it works

Overflow — water leaves the display tank into the sump, either through a drilled tank or an overflow box.
Mechanical section — filter socks or sponges catch debris first.
Biological section — ceramic rings, bio-balls or sponge house the filter bacteria.
Return pump — pushes water back up into the tank, completing the loop.

Designing for a power cut

The single biggest risk with a sump is flooding. When the return pump stops — a power cut, a fault — water keeps draining from the display down to the sump until the level drops below the overflow. The sump must have enough spare capacity to hold that back-flow without overflowing, or you get water on the floor.

Flow must also balance: the return pump should not out-pump the overflow, or the tank fills while the sump runs dry. Set the pump and overflow to match, and mark a maximum water line in the sump so you always leave enough room for the drain-back.

Maintaining a sump

Maintenance is straightforward but regular: clean or replace the filter socks, rinse the biological media occasionally in old tank water, and keep the return pump clear of debris. Because the sump evaporates more than a closed tank, top up with RO water to avoid hardness and pH creeping up. Check the water level, the pump and the overflow now and then, so small issues do not become floods.

Do you need one?

For most beginner freshwater tanks, a good hang-on-back or canister filter is simpler, quieter and safer. A sump is worth the complexity once you keep a large or reef tank and want maximum filtration out of sight. If you are new to the hobby, start with a canister filter — a sump can wait.

Noise and simplicity

One downside of a sump is noise: water cascading over the overflow and the return pump both hum, and in a quiet room that is noticeable. Careful design and a good-quality pump reduce it, but a closed canister filter will always be quieter. If silence matters where the tank sits, weigh that against the sump's benefits before committing.

Choosing what goes in it

Because a sump holds so much media, it lets you run more biological filtration than any hang-on filter. Fill it mainly with porous biological media, add a mechanical stage before it to trap debris, and keep chemical media like carbon optional. More media means a more stable tank, but only if it is rinsed occasionally so it does not clog and channel water around it.

Do I need a sump?

Only for larger or specialised tanks. Most freshwater tanks do fine with a canister or hang-on filter.

Can a sump flood my floor?

Possibly, if it is not designed to handle a power cut. Always leave sump capacity for water draining back.

Is a sump better than a canister?

It offers more volume and hides gear, but is more complex. Canisters are simpler and quieter.

Tools for this guide

Aquarium volume calculator

Sources and research notes

Guidance follows standard fishkeeping practice; sumps add volume and hide equipment but require careful design to avoid flooding.

Practical Fishkeeping

Written and edited by the Tankwise Editorial. Every guide answers one real fishkeeping question, is checked against the sources listed above, and is updated when guidance changes. We are an independent editorial desk, not veterinary or aquarium professionals — read how we research and update our guides.