A sump tank is a secondary container that sits below your main aquarium, housing your filtration equipment and adding water volume to the system. Building one yourself gives you full control over the layout and saves a significant amount compared to buying a pre-made unit. The process involves choosing the right container, dividing it into chambers with glass or acrylic baffles, and plumbing it to your display tank.
Choosing the Right Size
Your sump should hold no less than 25% of your display tank’s total water volume. A 100-gallon tank needs at least a 25-gallon sump, though bigger is always better. The extra water volume stabilizes temperature and chemistry, gives you room for equipment, and provides a buffer against evaporation.
There’s a critical sizing factor many beginners overlook: when the return pump shuts off (during a power outage, for example), water from the display tank will drain back into the sump through the overflow until it reaches the level of the overflow teeth or the siphon break. Your sump needs enough empty space above its normal waterline to hold all of that back-siphoned water without flooding. After you finish plumbing, test this by turning the return pump off and watching where the water level settles in the sump. If it’s close to the rim, you need a larger sump or to lower the normal operating water level.
Most DIY sumps start as standard glass aquariums, since they’re inexpensive and easy to modify. A 20-gallon long tank is a popular choice for systems in the 50 to 75-gallon range because its low, wide footprint fits neatly inside most aquarium stands.
Planning the Chamber Layout
A sump works by moving water through a series of chambers, each handling a different filtration job. The most common layout uses three sections:
- First chamber (drain side): Water enters here from the display tank. This is where you place mechanical filtration like a filter sock and your protein skimmer. Catching debris first prevents it from reaching the other chambers.
- Second chamber (middle): This section can serve as a refugium for growing macroalgae, which absorbs nitrates and phosphates. Alternatively, you can use it for chemical filtration media like carbon or bio-media.
- Third chamber (return): The return pump sits here, pushing clean water back up to the display tank. This section should be large enough that the water level doesn’t fluctuate dramatically from evaporation between top-offs.
There’s ongoing debate about whether the refugium should come before or after the skimmer. In practice, the order makes very little difference to filtration performance. The amount of nutrient removal on any single pass through the sump is tiny, so neither section starves the other. The one rule that does matter: put mechanical filtration (filter socks or foam) at the inlet so large debris gets caught immediately.
Building and Installing Baffles
Baffles are sheets of glass or acrylic siliconed inside the sump to create separate chambers and control water flow. The key design element is the bubble trap, a set of three baffles arranged so water is forced under one, over the next, and under the last. This path traps microbubbles and prevents them from reaching your return pump and getting blasted into the display tank.
For baffle spacing, 1 to 1.5 inches between each panel works well. A common trick is to lay a piece of lumber (the narrow side of a 2×4, which is actually 1.5 inches) between the baffles as a spacer while the silicone cures. The center baffle in a bubble trap sits raised off the bottom of the sump, and that gap should match the spacing between the baffles. So if your baffles are 1.5 inches apart, the center baffle should be raised 1.5 inches off the floor.
Setting Baffle Heights
The height of your first baffle (between the skimmer chamber and the middle section) needs to match the operating water depth your protein skimmer requires. Most internal skimmers specify a narrow range, sometimes as tight as 1 inch. Check your skimmer’s manual and set the first baffle height so the water level in that chamber falls within the recommended depth. Getting this wrong affects air draw and skimmer performance noticeably.
The over-under-over pattern of the bubble trap means the first and third baffles in the trap are taller (water goes under them) and the middle one is shorter and raised off the floor (water goes over it). A common approach is making the outer baffles reach about an inch below the sump’s rim, with the center baffle 2 to 3 inches shorter than the outer ones.
Gluing Baffles in Place
Use 100% silicone to attach the baffles. All silicones are reef-safe once fully cured. The real concern is letting them cure completely before adding water. Published cure times on the label are based on a thin bead, usually 1/8 to 1/4 inch. When you’re laying down thick lines to seal a baffle, cure time increases substantially, sometimes to one or two weeks. Give it at least 72 hours for thin applications and a full week for thicker joints to be safe.
For glass-to-glass bonding, higher-end silicones from Dow Corning (such as DC 735 or DC 999A) outperform budget options like GE Silicone I in adhesion strength. If you’re bonding glass to acrylic, a neutral-cure silicone works, but the bond will always be weaker than glass-to-glass. For acrylic-to-acrylic, solvent cement designed for acrylic creates a much stronger weld than silicone.
Drilling and Installing Bulkheads
Your display tank needs holes for water to flow down to the sump and return back up. If your tank didn’t come pre-drilled, you can drill tempered glass alternatives or have a glass shop do it. The bulkhead fitting passes through the hole and creates a watertight seal.
Correct orientation matters. The flange and gasket sit flush on the inside of the tank, and the threaded body passes through to the outside. You then thread the retaining nut onto the outside, compressing the gasket against the glass. Hand-tighten firmly, then give it a quarter turn more. Over-tightening can crack glass.
For the drain line (display to sump), a 1-inch bulkhead handles roughly 600 gallons per hour by gravity flow. That’s sufficient for most tanks up to about 120 gallons when paired with an appropriately sized return pump. Many hobbyists install a second drain as an emergency backup in case the primary clogs.
Plumbing the Drain and Return
Use PVC pipe to connect the display tank’s drain bulkhead to the sump’s inlet chamber, and from the return pump back up to the display. Schedule 40 PVC is standard, and connections can be either cemented (permanent) or use slip fittings and unions for sections you want to disassemble later. Unions at the pump and at any point entering the sump are especially helpful for maintenance.
The drain line should include a valve to control flow rate, and the pipe entering the sump should terminate below the waterline or into a filter sock holder to minimize splashing and salt creep. For the return line, drill a small hole (about 3/16 inch) near the waterline of the return nozzle in the display tank. This siphon break allows air into the line when the pump shuts off, stopping back-siphoning before the sump overflows.
Sizing the Return Pump
The general target is a return pump that cycles your total system volume through the sump 2 to 10 times per hour. For modern reef tanks, most hobbyists land in the 3x to 5x range after accounting for head pressure, since powerheads inside the display handle most of the circulation work.
Head pressure is the resistance your pump fights against to push water upward and through fittings. Calculate it by adding up these factors:
- Vertical rise: Every 1 foot of height from the pump to the return nozzle equals 1 foot of head pressure.
- 90-degree elbows: Each one adds 1 foot of head pressure.
- 45-degree elbows: Each one adds 0.5 feet of head pressure.
- Horizontal runs: Every 10 feet of horizontal pipe adds 1 foot of head pressure.
A typical setup with 4 feet of vertical rise and three 90-degree elbows gives you 7 feet of head pressure. Look at the pump’s flow curve chart (every manufacturer publishes one) and find the flow rate at that head pressure, not the pump’s maximum rating. A pump rated at 800 GPH at zero head might only deliver 400 GPH at 7 feet, which could be exactly right for a 100-gallon system aiming for 4x turnover.
Adding a Refugium Section
If your middle chamber is a refugium, you’ll grow macroalgae there to export nutrients from the water. Chaetomorpha (a tangled ball of green algae) is the most popular choice because it grows fast, doesn’t go sexual and crash, and is easy to harvest by simply pulling out handfuls when it gets thick.
Macroalgae needs light to grow. A dedicated refugium light with emphasis on red-spectrum LEDs drives the fastest growth. Full-spectrum white lights also work, just slightly less efficiently. Many hobbyists run the refugium light on a reverse schedule from the display (refugium lit at night, display lit during the day), which helps stabilize pH by keeping photosynthesis running around the clock.
Flow through the refugium should be gentle enough that the algae isn’t tumbling violently but strong enough to keep detritus from settling. The natural flow rate from your return pump cycling through the sump usually provides this without any additional equipment.
Testing Before Going Live
Fill the entire system with freshwater first. Run the return pump and check every joint, bulkhead, and baffle for leaks. Then turn off the pump and watch the sump. The water draining back from the display must not exceed the sump’s capacity. If it gets too close to the top, lower the return nozzle’s position in the display or raise the siphon break hole. Only after this test passes should you add saltwater and begin cycling the tank.