An aquarium sump is a separate tank, usually hidden in the cabinet below the main aquarium, that water flows down into, gets filtered, and is pumped back up. It expands filtration capacity, hides equipment, and increases the total water volume — making it popular for large freshwater tanks and almost universal in marine reef setups.
Here is what a sump does and whether you need one.
- A sump is a hidden tank below the main aquarium that expands filtration and hides equipment.
- It increases total water volume, giving more stable parameters.
- Sumps are standard for marine reef tanks and beneficial for large freshwater tanks.
- Small and medium community tanks don't need one — a canister filter is enough.
How a sump works
Water overflows from the main tank down into the sump through a drain, passes through filtration compartments (mechanical, biological, sometimes a protein skimmer or refugium), and a return pump sends the clean water back up. It runs continuously as a closed loop.
What a sump gives you
- More filtration — far more space for media than any canister filter.
- Hidden equipment — heaters, skimmers and reactors sit out of sight in the sump, not the display tank.
- More water volume — a bigger total volume means more stable water parameters.
- A refugium option — a compartment for beneficial macroalgae or a place to grow live food.
Do you need one?
Marine reef tanks: effectively yes — a sump is standard for skimmers and stable chemistry. See marine tank setup.
Large freshwater tanks (300 litres+): beneficial but optional — a good canister filter is enough for most community tanks.
Small and medium tanks: no — a sump is overkill; an external canister filter does the job.
Frequently asked questions
What is an aquarium sump?
It is a separate tank, usually in the cabinet below the main aquarium, that water drains into for filtration before being pumped back up. It expands filtration, hides equipment and increases water volume.
Do I need a sump for my aquarium?
Marine reef tanks effectively need one; large freshwater tanks benefit but can use a canister filter; small and medium community tanks do not need a sump at all.
What is the difference between a sump and a refugium?
A sump is the whole below-tank filtration system. A refugium is one compartment within a sump used to grow beneficial macroalgae or live food.
— Jaqline Smalling, FishForever
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