Why a Sump Makes Your Reef Tank Better

A sump is a secondary tank, usually placed in the cabinet below your display tank, that houses equipment and adds total water volume to your system. It is not strictly required, but it transforms a reef tank in several meaningful ways.

More water volume means slower, more stable parameter swings. A heater failure, a missed water change, or a spike in feeding all have less impact when your system holds 80 gallons instead of 50. Stability is the single most valuable quality a reef tank can have, and a sump is one of the most effective ways to buy it.

A sump also moves equipment out of your display. Heaters, protein skimmers, return pumps, and refugium sections all live in the sump, leaving your display tank clean and uncluttered. Maintenance becomes faster because everything is accessible in one place underneath the tank rather than scattered inside a visible aquarium.

Sizing Your Sump

A sump should hold at least 20 to 30 percent of your display tank volume. For a 75-gallon display, a 20 to 30-gallon sump is the minimum. Bigger is always better within the space constraints of your cabinet.

There is one critical sizing rule: your sump must be large enough to handle the drain-back volume when power goes off. When your return pump stops, water siphons back from the display tank through the return line until the siphon breaks. If your sump is too small to hold that extra water, it overflows. Calculate your drain-back volume by unplugging your return pump and watching how much water drains back into the sump, then make sure your sump has that much free headspace.

Standard sump sizes that fit typical tank stand cabinets:

  • 20-gallon long (30” x 12” x 12”) for tanks up to 60 gallons
  • 30-gallon (36” x 12” x 16”) for tanks 60 to 90 gallons
  • 40-gallon breeder (36” x 18” x 16”) for tanks 90 gallons and up

The Standard Three-Chamber Layout

Most reef sumps divide the internal volume into three sections using baffles (internal walls that water flows over or under).

Chamber 1: Drain/Filter Sock Chamber

Water enters the sump here from your display tank drain line. This is where your filter sock or filter roller sits to catch particulate matter before it reaches the rest of the sump. Keep this chamber easy to access since the filter sock needs cleaning every few days.

Some hobbyists skip the filter sock entirely and let detritus settle naturally, cleaning the first chamber with a siphon during water changes. Both approaches work. Filter socks trap more waste but require regular maintenance.

Chamber 2: Equipment/Refugium Chamber

The middle chamber is where your protein skimmer sits. It can also house a refugium section with chaeto and a refugium light, a deep sand bed for additional denitrification, or both if the sump is large enough.

If you want a refugium, divide the middle chamber in half with a partial baffle: one half for the skimmer, one half for the refugium. Keep the skimmer and refugium sections separated so that the turbulent water from the skimmer does not disrupt chaeto growth.

Chamber 3: Return Chamber

The final chamber holds your return pump, heater, and ATO (auto top-off) sensor. Water leaves this chamber via the return pump, which pushes it back up to the display tank through a return line.

Keep this chamber relatively full. If the water level drops too low, your return pump runs dry and burns out. An ATO system that automatically adds fresh RODI water to maintain a consistent level in the return chamber solves this problem passively.

Plumbing the Drain: Overflow Options

Getting water from your display tank down to the sump requires an overflow. There are three main approaches.

Built-In Overflow (Bean Animal or Herbie)

The cleanest and quietest option. Your tank is drilled, and an internal overflow box captures surface water and sends it down to the sump through pipes in the stand. The “bean animal” uses three pipes (full siphon main drain, emergency drain, and open channel drain) and runs nearly silent. The “Herbie” uses two pipes and is almost as quiet with simpler plumbing.

Drilled overflows are standard on purpose-built reef tanks. If you are buying a new aquarium for a serious reef build, get one that is pre-drilled or drill it before adding water.

Hang-On-Back Overflow Box

A HOB overflow siphons water over the tank rim into an internal box, which then drains into a tube going down to the sump. No drilling required. The risk is losing the siphon during a power outage, which stops the drain and can overflow the display if the return pump continues running. Dual-standpipe HOB overflows (like the CPR CS90) have built-in siphon protection and are safer than single-standpipe designs.

Durso or Standpipe

In drilled tanks, the standpipe design affects noise level and drain rate. A Durso standpipe (a capped vertical pipe with a small air hole at the top) reduces gurgling significantly compared to a bare open pipe. A stockman standpipe goes a step further with an air intake that nearly eliminates noise.

The Return Pump

Your return pump pushes water from the sump back up to the display tank. Size it to turn over your total system volume (display plus sump) 5 to 10 times per hour, accounting for head pressure (the resistance from pushing water upward through the return line).

A pump rated at 1200 GPH at zero head pressure might deliver 800 GPH after running up 4 feet of vertical pipe and through an elbow or two. Check manufacturer head pressure charts when sizing.

For most home reef tanks in the 50 to 120-gallon range, the Sicce Syncra SDC and the Reef Octopus VarioS are well-regarded variable-speed return pumps that let you dial in the exact flow rate you want.

A ball valve on the return line lets you reduce flow without changing the pump. Install one.

Return Line Configuration

The return line from your sump runs up through the stand and into the display tank. Two return nozzles aimed slightly downward from opposite ends of the tank give better surface agitation and flow distribution than a single central return.

Install a check valve on the return line to reduce drain-back when power goes off. Check valves are not perfect, they slow the drain-back rather than stopping it, but they buy you a smaller required sump headspace.

Loc-Line flexible tubing makes aiming return nozzles easy without cutting hard pipe. Loc-Line 1/2-inch modular hose kits are inexpensive and available in most aquarium stores.

Equipment Placement Tips

Put your heater in the return chamber where the water level is most stable and the flow is consistent. Heaters that experience fluctuating water levels (as in the drain chamber when flow varies) cycle on and off erratically and wear out faster.

Mount your skimmer in its own chamber away from return pump turbulence. Turbulent water entering the skimmer reaction body disrupts foam formation. A calm, steady water surface in the skimmer chamber dramatically improves skimmer performance.

If you run a refugium section, light it on a reverse schedule from your display tank. Lights on at night in the refugium stabilize your system pH by consuming CO2 during the hours when your display lights are off and photosynthesis has stopped.

FAQ

Does my reef tank need a sump? No, but it helps enormously. All-in-one tanks with built-in rear filter chambers offer a middle ground. A true sump adds more volume, more equipment space, and more flexibility than any AIO compartment.

Can I build my own sump? Yes. A DIY sump from a standard glass aquarium with silicone baffles and a coat of black spray paint on the outside is functionally identical to a commercial sump and costs a fraction of the price. Many hobbyists use a used 20-gallon long aquarium as their first sump.

How do I stop my sump from making noise? Most sump noise comes from the drain line. A properly configured Herbie or bean animal overflow runs near-silently. If you have an existing noisy drain, adding a Stockman standpipe insert and adjusting the drain valve usually solves it. Submerging the drain outlet below the water surface in the sump also eliminates splash noise.

What is the best water level to maintain in the return chamber? Keep the return chamber at least one-third full at all times to prevent the pump from running dry. An ATO system tied to a float switch or optical sensor in the return chamber keeps the level consistent automatically.

Do I need a check valve on the return line? Recommended but not essential if your sump is sized for full drain-back. A check valve reduces the drain-back volume and gives you more margin for error. Use a spring-loaded check valve rather than a flap style, which can stick open.