A sump is a secondary tank, typically mounted below the display tank, that handles your filtration, equipment, and water volume out of sight. In the context of aquarium life support systems, it's the backbone of water quality management for serious freshwater and saltwater setups. If you're running a coral reef, a large fish-only tank, or any system where you want equipment accessible without disturbing fish, a well-designed sump makes every part of tank maintenance simpler and more reliable.

This guide covers how sump-based life support systems are designed, what each compartment does, how to size a sump for your display tank, what equipment belongs inside, and how to avoid the common mistakes that turn a sump into a headache. I'll use specific products and dimensions so you can build or buy with real numbers in hand.

What a Sump-Based Life Support System Actually Does

A sump serves four functions simultaneously: mechanical filtration (trapping solids), biological filtration (processing ammonia and nitrite), chemical filtration (running carbon, phosphate media, or other reactors), and equipment housing (heaters, return pumps, dosing heads, skimmer). All of these happen out of the display tank, which keeps the main display clean and uncluttered.

The display tank's overflow drains water into the sump by gravity. A return pump pushes water back up to the display. Everything else in the system, including the skimmer, reactor, and refugium, runs off this single gravity-drain/return-pump loop.

The immediate practical benefit is water volume. A 75-gallon display tank with a 30-gallon sump is actually a 105-gallon system. More water volume means more stable parameters. Temperature swings, salinity shifts from evaporation, and pH fluctuations all get dampened by the larger total water mass.

Designing the Sump: Compartment Layout

Most functional sumps have three to four compartments separated by baffles. Here's how a typical layout works from left to right as water flows through.

Skimmer Section

Water from the overflow enters the first compartment, which is the skimmer section. This compartment stays at a consistent water level, which is critical for protein skimmer operation. Most skimmers are calibrated for a specific water depth, typically 7 to 10 inches. Fluctuating water levels in this section cause the skimmer to hunt and produce inconsistent foam.

A standard build uses the first section as 25 to 30 percent of total sump volume. For a 30-gallon sump (48"x13"x16"), that's about a 12-inch-wide first section.

Refugium or Biological Section

The middle section is where most of the biological action happens. For reef tanks, this is typically a refugium growing Chaeto (Chaetomorpha) algae under a grow light. Chaeto absorbs nitrate and phosphate directly, competing with nuisance algae in the display. For freshwater tanks, this section can house a fluidized sand filter, a trickle tower, or just biological media in a chamber.

A refugium light on a reverse cycle (lit when the display is dark) helps stabilize pH by keeping photosynthesis happening 24 hours a day. The AI Fuge 16HD is a popular refugium LED, drawing only 16W but producing enough PAR for dense Chaeto growth.

Return Section

The final section houses the return pump. This is where water exits the sump back to the display tank. Size this section to accommodate the return pump body, heaters, and a water level that stays consistent even when evaporation occurs. Most builders make the return section at least 10 inches wide to house equipment comfortably.

Sizing the Sump and Overflow

A sump should hold at least 20 to 30 percent of the display tank's water volume. For a 100-gallon display, that means a 20 to 30-gallon sump minimum. Bigger is better for stability, limited only by cabinet space.

The overflow rate needs to match your return pump's output. A common mistake is running a 1,000 GPH return pump through an overflow rated for 600 GPH. The sump water level rises continuously until it overflows onto the floor. Always match overflow capacity to return pump flow, then add 20 percent buffer.

Overflow Types

Hang-on-back overflows like the CPR CS90 (rated 900 GPH) are the easiest retrofit for tanks without a built-in overflow. They use a siphon to drain water over the back wall. The downside is that siphons can break during power outages and need priming to restart.

Bulkhead overflows drilled through the tank bottom or back wall are more reliable. A standard 1.5-inch bulkhead flows roughly 750 to 900 GPH. A 2-inch bulkhead flows 1,200 to 1,500 GPH. Most professional aquarium life support installations use dual bulkheads for redundancy.

Return Pumps and Flow Rate

For a sump-based life support system, you want the total system turnover (display volume + sump volume) to cycle 5 to 10 times per hour through the sump. A 100-gallon display with a 30-gallon sump is 130 total gallons. At 5x turnover, you need a 650 GPH return pump after accounting for head pressure losses.

Head pressure reduces pump output based on how high the pump must push water. Every 10 vertical feet costs roughly 10 percent of pump output for most centrifugal pumps. A pump rated 1,000 GPH at 0 head might only deliver 700 GPH pushing water 6 feet up to the display.

The Sicce Syncra 3.0 (rated 793 GPH at 0 head) is a reliable DC return pump option for tanks up to about 120 gallons. The Reef Octopus DC-4500 handles larger systems up to 200 gallons. DC pumps are preferred over AC pumps for return duty because they let you dial in exact flow and adjust for seasonal changes.

Equipment That Lives in the Sump

Beyond the return pump, a well-configured sump houses multiple pieces of life support equipment.

A protein skimmer belongs in the first section. The Bubble Magus Curve 5 handles tanks up to 140 gallons and fits neatly in sumps with a minimum water depth of 7.5 inches. For larger systems, the Reef Octopus Classic 150INT handles up to 175 gallons.

Heaters go in the return section where they're always submerged. Two heaters in parallel (controlled by an external controller like the Inkbird ITC-308) provides redundancy and more even heat distribution.

A media reactor mounted outside the sump but plumbed into it handles carbon or GFO (granular ferric oxide) for phosphate control. The Two Little Fishies PhosBan Reactor 150 is compact and effective for tanks up to 150 gallons.

For more comprehensive equipment options across all filtration and life support categories, check out our Best Aquarium Equipment guide and the Top Aquarium Equipment roundup.

Common Sump Mistakes and How to Avoid Them

The most common mistake is insufficient sump volume below the overflow intake. When power goes out, water siphons from the display until the water level drops to the bottom of the overflow. All that water drains into the sump. If the sump can't hold the drain-down volume, it overflows onto the floor. Calculate your drain-down volume by measuring how many gallons the display tank holds from the overflow teeth to the bottom of the tank, then ensure your sump has at least that much open space above its normal operating level.

Bubble problems come from return lines disturbing the water surface in the return section. Route the return line below water level and angle it toward the sump wall to reduce turbulence. Micro-bubble injection into the display tank typically traces back to a skimmer neck being too low in the first section or a return pump churning air from an overly shallow operating level.

Noise from the overflow is reduced by fitting a Herbie-style drain configuration: one full-siphon drain pipe and one emergency overflow. The full-siphon drain runs silently and the emergency overflow only flows if something blocks the main drain. This eliminates the gurgling noise that open drain pipes produce.

FAQ

How big should my sump be relative to my display tank? Aim for 25 to 40 percent of the display tank volume. A 120-gallon display works well with a 30 to 48-gallon sump. The larger the sump, the more stable your water parameters and the more room you have for equipment.

Do I need a refugium in my sump? Not strictly, but for reef tanks a refugium growing Chaeto algae significantly reduces nitrate and phosphate with no running cost beyond a small LED light. For heavily stocked freshwater tanks, a biological media chamber in a similar position does the same job without the algae.

What happens to the sump if the power goes out? Water from the display tank siphons through the overflow until the water level drops to the bottom of the overflow intake. This drain-down volume flows into the sump. Your sump must have enough freeboard to accept this water without overflowing. Most builders leave 4 to 6 inches of air space above the normal sump water level to accommodate drain-down.

Can I run a sump on a freshwater planted tank? Yes, and it works very well. A sump adds water volume, keeps equipment out of the display, and you can grow floating plants like hornwort in the refugium section for additional nutrient export. The only caveat is that CO2 injection is less efficient with a sump because the return pump agitates the surface and off-gases CO2 faster. A spray bar return aimed below the surface reduces this loss.