What Is a Sump?
A sump is a second tank below your display that filters water and hides equipment. Learn how the overflow and return loop works and why reefers use one.
A sump is a second tank, usually tucked in the cabinet below your display, that shares water with the main aquarium so you can filter the water and hide equipment out of sight. In plain terms, it is a hidden basement for your tank where all the messy gear lives.
Sumps are most common on saltwater and reef tanks, but they work on freshwater systems too. They solve three problems at once: they add water volume for stability, they create space for equipment, and they give devices like protein skimmers a steady water level to run in.
What a sump does for your tank
A sump is not a single piece of gear so much as a flexible workspace. Because it is just another tank below the display, you can fit whatever your system needs into it.
- Houses equipment like heaters, the return pump, a protein skimmer, reactors, and probes, all out of view.
- Adds water volume, and more total water means slower, smaller swings in temperature and chemistry.
- Holds filtration media in dedicated chambers, from filter socks and floss to biological media.
- Creates a refugium space where macroalgae or live rock can grow to export nutrients.
- Provides a top-off point where evaporated water is replaced without touching the display.
That extra water volume is a real advantage. More water is more forgiving, which is the same reason bigger tanks are easier for beginners. You can estimate your full system volume, sump included, with our aquarium volume calculator.
How the overflow and return loop works
A sump runs on a simple, continuous loop of water moving down by gravity and back up by pump.
- An overflow box on the display tank sets the maximum water level. When water rises above it, the excess drains.
- Water falls through drain pipes down into the sump below.
- In the sump, water passes through filtration chambers, mechanical media, the skimmer, and the heater.
- A return pump in the final chamber pushes the cleaned water back up to the display.
- The returning water raises the display level, it overflows again, and the cycle repeats.
The whole system self-balances as long as the return pump runs. The flow rate of that pump sets your turnover, and you can size it with our filter turnover calculator.
Anatomy of a typical sump
| Section | What it holds | Purpose |
|---|---|---|
| Inlet / first chamber | Filter sock or floss, skimmer | Catch particles and export waste first |
| Middle chamber | Biological media or refugium | Process ammonia and grow nutrient-exporting algae |
| Return chamber | Return pump, heater, probes | Send clean, warmed water back to the display |
Planning for power outages
The most important sump skill is planning for the pump shutting off. When power fails, the display stops overflowing, but water in the drain lines and a siphon back through the return line will drain down into the sump. Your sump must have enough empty space at the top to hold all of that water without overflowing onto the floor.
Aquarists handle this by marking a safe maximum fill line in the sump and drilling a small anti-siphon hole in the return line just below the surface to break the backflow. Always run a power-off test before livestock go in. A few minutes of dry-run testing prevents a flooded room.
Is a sump right for you?
If you keep a reef tank, a sump is close to standard equipment, and it pairs naturally with a protein skimmer, which needs the constant water level a sump provides. Freshwater keepers can run beautifully with a canister or hang-on-back filter and skip the sump entirely. For the full picture of how a sump fits into a marine build alongside RODI water and salt mixing, see our saltwater aquarium for beginners guide.
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Frequently Asked Questions
What is a sump in an aquarium?
A sump is a second tank, usually hidden in the cabinet below your display, that is plumbed to share water with the main tank. Water overflows from the display down to the sump, gets filtered and conditioned there, and a return pump sends it back up. It effectively adds water volume and gives you a roomy, out-of-sight place to house equipment like heaters, skimmers, and media.
Do I need a sump for my tank?
No, plenty of healthy tanks run with no sump at all, especially freshwater setups using hang-on-back or canister filters. A sump is most valued on saltwater and reef tanks because it hides gear, adds water volume for stability, and gives a protein skimmer a steady water level to work in. If you want a clean display with no visible equipment and more room to grow, a sump is a worthwhile upgrade.
How does the overflow and return system work?
Water flows in a continuous loop. An overflow box on the display tank lets water drain down through pipes to the sump whenever the level rises above a set point. After passing through the sump chambers, a return pump pushes water back up to the display through a return line. As long as the pump runs, the loop stays balanced and the display level holds steady.
What happens to a sump if the power goes out?
When the return pump stops, the display stops overflowing, but water already draining and water in the return line siphon back down into the sump. You must leave enough empty space in the sump to catch all that water, or it overflows onto the floor. Aquarists set a safe maximum fill level and add anti-siphon holes in the return line to limit backflow. Always test this with a power-off drill before adding livestock.
Can I put a heater in the sump?
Yes, and it is one of the main reasons people love sumps. Moving the heater, skimmer, and other gear into the sump keeps the display clean and uncluttered. The constant water flow through the sump ensures the heater warms the whole system evenly. Just make sure the sump section holding the heater always stays full of water, since a heater exposed to air can overheat and crack.
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