Sump: What It Is and Why Reef Keepers Use One
A sump is a second tank, usually positioned below the display in a cabinet stand, that water drains into and gets pumped back up from. Rather than cramming filtration equipment, a protein skimmer, dosing pumps, and a heater into the display tank itself, a sump moves all of it out of sight and out of the way, leaving the main tank free of visible hardware.
How a Sump Is Plumbed Into a Tank
Water leaves the display through an overflow box or a drilled bulkhead near the top of the tank and drains down into the sump by gravity. Inside the sump, the water usually passes through a series of baffles, walls that force it to flow up and over or under before reaching the return section, which slow the flow enough to let air bubbles rise out before a return pump sends the water back up to the display. A return pump sized to the plumbing and head height keeps water moving continuously between the two tanks, and the drain and return flow rates need to be reasonably matched so the sump neither overflows nor runs dry.
Why Reef Keepers Use Sumps
Beyond hiding equipment, a sump adds total system water volume, which dilutes waste and buffers parameter swings compared to running the same tank without one. It also gives a dedicated space for a protein skimmer, refugium, dosing pumps, and a heater, all serviceable without disturbing the display, and it keeps evaporation and top-off changes confined to the sump’s return section rather than visibly moving the display’s water line.
A sump section can also double as a refugium, growing macroalgae or housing extra live rock to add biological filtration and export nutrients, without taking up any space in the display itself.
Setting Up a Sump
A sump is usually sized as a fraction of display volume, often somewhere around a quarter to a third of it, though the real constraint is cabinet footprint and how much of that volume the baffles, skimmer, and refugium section take up. Overflow capacity matters more than most first-time reefers expect: the overflow box or drilled bulkhead needs to handle more flow than the return pump actually pushes, so a power outage or a pump surge never sends more water into the sump than it can hold.
Most setups also include a permanent water-level mark in the sump’s return section, which makes topping off for evaporation a quick visual check rather than a guess, and leaves enough freeboard above that mark to absorb the extra volume that drains down when the return pump is switched off.
Common Sump Mistakes
An undersized overflow relative to the return pump is one of the most common and most damaging mistakes, since it risks the display overflowing rather than the sump. Sizing the overflow with real headroom above the pump’s rated flow avoids this. Skipping bubble-trapping baffles is another frequent shortcut that shows up later as a stream of microbubbles in the display, which is harder to fix after the sump is already plumbed in than to build in from the start.
It’s also worth planning for what happens if the return pump fails or gets unplugged: without a check valve or a properly sized sump return section, water can siphon back from the display into the sump and overflow it. A tall enough return section, or a check valve on the return line, is the usual fix.
FAQ
What is a sump for an aquarium?
It’s a second tank, typically housed in the stand below the display, that holds filtration equipment, a protein skimmer, and dosing gear out of sight while adding total system water volume.
What is a sump in an aquarium, in terms of how it connects to the display?
Water drains from the display down into the sump through an overflow or drilled bulkhead, gets filtered and processed there, and a return pump sends it back up to the display in a continuous loop.
Do I need a sump for a reef tank?
Not strictly. Plenty of nano and all-in-one reef tanks run without one, but a sump makes equipment placement, dosing, and refugium space far more practical on anything beyond a small system, which is why most serious reef setups use one.
What happens if my return pump fails?
Water stops being pumped back to the display and settles at its resting level in the sump. The risk runs the other way too: without a check valve or enough sump return capacity, water already in the display can siphon back down and overflow the sump. Sizing the return section with this in mind avoids it.
Can a sump be used in a freshwater tank?
Yes. Sumps are more strongly associated with marine and reef setups because of skimmer and dosing equipment, but the same benefits apply just as well to a freshwater system: extra water volume, hidden filtration, and a stable display water level.
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