A sump is a secondary tank (usually located in the cabinet below your display tank) that holds your filtration equipment and increases your total water volume. The core equipment you need for an aquarium sump includes a return pump, protein skimmer, biological filter media, a refugium light if you're growing macroalgae, and a float switch for your auto top-off system. You might also add heaters, reactors, and dosing equipment depending on how complex your system is.
This guide covers each piece of sump equipment in detail, how they work together, what sizes to buy for different tank volumes, and how to set up the whole system without making the common mistakes that new sump builders encounter.
Why Run a Sump?
Before covering equipment, it helps to understand why sump systems are worth the effort. The two biggest benefits are increased water volume and equipment consolidation.
More water volume means slower parameter swings. A heater failure that would crash a 50-gallon display-only system has less immediate impact when you've got an additional 20 gallons of water in the sump buffering temperature changes. The same applies to salinity, pH, and chemical spikes.
Equipment in the sump is hidden from view. Protein skimmers, heaters, reactors, and wiring all live below the tank rather than cluttering the display. The result is a cleaner, more natural-looking aquarium.
Sumps also give you a place to grow macroalgae (chaeto, caulerpa) that exports nutrients from the system, reducing nitrate and phosphate without water changes.
Return Pump
The return pump sits in the sump and pushes water back up to the display tank. It's the heart of the whole circulation system. Your overflow brings water down from the display into the sump by gravity, and the return pump pushes it back up.
Sizing the Return Pump
Return pump flow rate should match roughly 5 to 10 times the display tank volume per hour. For a 50-gallon display tank, that's 250 to 500 gallons per hour (GPH) at the pump head. But head pressure matters. Every foot of vertical rise and every 90-degree elbow in the return plumbing reduces actual flow. A pump rated at 500 GPH free-flowing might only push 300 GPH when pumping water 5 feet up through elbows.
The Sicce Syncra Silent 3.0 is rated at 793 GPH free flow and is popular for 40 to 75 gallon systems. It runs very quietly and has a 2-year warranty. At around $80, it's a mid-range option with a strong reliability record.
The Eheim Universal 1250 runs at about 317 GPH and works well for smaller systems (20 to 40 gallon displays) or as a supplemental pump. It's one of the quietest submersible pumps available.
For larger systems (75 to 150 gallon displays), the Reef Octopus VarioS 4 DC Pump is controllable by a dial or aquarium controller, letting you fine-tune flow without swapping equipment. DC (direct current) pumps are more energy-efficient and heat the water less than older AC pumps.
External vs. Submersible Return Pumps
Submersible pumps (Sicce, Eheim, Reef Octopus) sit in the sump water directly. They're easier to install but add heat to the water. External pumps (like the Reeflo Barracuda) mount outside the sump on a bulkhead fitting. They run cooler and are easier to service, but installation is more involved and they're louder.
For most home reef setups, submersible DC pumps are the standard choice.
Protein Skimmer
The protein skimmer goes in the sump's first chamber (the intake section where raw water from the overflow arrives). This position lets the skimmer process unfiltered water with the highest organic load before it passes through other filtration stages.
For sump skimmer sizing in a typical reef setup, target a skimmer rated for 1.5 to 2 times your total system volume (display plus sump). A 50-gallon display with a 15-gallon sump means 65 total gallons, so a skimmer rated for 100 to 130 gallons gives you real-world performance that matches the actual bioload.
The Reef Octopus Classic 110-S is a standard recommendation for systems up to 105 gallons. It operates in 6 to 8 inches of water depth and produces consistent dark skimmate with minimal adjustment after initial setup.
The Bubble Magus Curve A5 handles systems up to 140 gallons, sits in the sump's first chamber, and uses a needle wheel pump design that breaks water into fine bubbles efficiently. It costs around $130 and is one of the better budget-to-mid options.
Refugium Section and Macroalgae
Many sump designs include a middle section called a refugium where macroalgae (usually chaeto, short for Chaetomorpha) grows under a dedicated light. The algae absorbs nitrate and phosphate and exports them from the system when you harvest it (periodically pulling out a handful and discarding it).
The Kessil H80 Refugium Light is purpose-built for this application. It produces a deep red and blue spectrum optimized for algae growth rather than coral viewing, and it mounts cleanly over sump sections with a magnetic arm. It runs about $80.
Budget alternative: the Innovative Marine Nuvo SR 60 Macroalgae Light at around $50 produces a pink-red spectrum that drives chaeto growth effectively.
Run the refugium light on a reverse photoperiod from the display (when display lights are off, refugium light is on). This stabilizes pH by having the algae consume CO2 during the night when the display's corals and fish are producing it.
The refugium section also works as a place to raise copepods and amphipods that migrate into the display and provide live food for mandarin dragonets and other pod-eating fish.
Biological Filtration Media
The sump's return section typically holds biological filter media before water gets pumped back to the display. Choices include:
Live rock rubble: Chunks of live rock placed in a mesh bag provide a colonization surface for nitrifying bacteria. Free if you have spare rubble from setting up the display tank.
Ceramic biological media: Products like Seachem Matrix or Marinepure Bio Media offer high surface area in a compact form. A 500ml container of Seachem Matrix handles the bioload from a moderately stocked 50-gallon system.
Biological filter socks: The sump's return pipe from the overflow typically passes through a filter sock (felt or mesh bag) before entering the sump. This removes large particulate matter before it settles. 200-micron felt socks are standard. Clean or replace them every 3 to 5 days; dirty filter socks become anaerobic and produce nitrate rapidly.
Auto Top-Off System
Saltwater evaporates and leaves salt behind, raising salinity over time. An auto top-off (ATO) system automatically adds fresh RODI water to the sump to replace evaporation losses and maintain stable salinity.
The AutoAqua Smart ATO Micro uses an optical sensor in the sump and a small pump to add water when the level drops. It costs about $70 and is one of the more reliable compact ATO systems. The dual-sensor design prevents pump runaway if a sensor gets dirty or stuck.
The Tunze Osmolator 3155 is a premium ATO ($135) with a float switch backup sensor in addition to the primary optical sensor. It's been the industry standard for reliability for over a decade.
Install the ATO reservoir in or near the cabinet below the tank. A 5-gallon bucket works for most systems. For larger setups with faster evaporation, a 15 to 20 gallon Brute trash can serves as the RODI reservoir.
For more on aquarium equipment selection, our guide to the best aquarium equipment covers sump components alongside filtration, lighting, and other gear. The top aquarium equipment roundup includes detailed comparisons of return pumps and protein skimmers.
Heater Placement in the Sump
Putting heaters in the sump rather than the display tank keeps them out of sight and protects the display from heater accidents. The return pump's flow brings heated water into the display, distributing temperature evenly.
Two smaller heaters in the sump (rather than one large one) provide redundancy. If one fails in the "on" position, the second heater alone isn't powerful enough to overheat the system, giving you time to notice before temperatures reach dangerous levels.
Place heaters in the sump's last section (return section) so that the pump circulates warm water efficiently throughout the system.
FAQ
What size sump do I need for a 75-gallon display tank? A general guideline is that the sump should be at least 20 percent of the display tank volume. For a 75-gallon display, a 20-gallon sump is the minimum, and a 30-gallon or larger sump is better. Larger sumps increase total system water volume and give you more room for equipment. The limiting factor is usually the cabinet under the tank. Measure the cabinet interior dimensions before buying a sump.
How do I prevent the sump from overflowing during a power outage? The display tank's overflow drains by gravity when the return pump is running. When power goes out, the return pump stops and water in the return pipe and the display tank above the overflow level drains into the sump by gravity. Design your system so the sump can hold the "drain-down" volume. Run the return pump, then turn it off and watch how much extra water enters the sump. The sump water level should rise but not overflow.
Do I need a refugium or can the sump just hold filtration equipment? You don't need a refugium. A simple sump with a protein skimmer, biological media section, and return pump is fully functional. Refugiums add the benefit of macroalgae nutrient export and copepod cultivation but require an extra light and periodic harvesting maintenance. Many reefers run sumps without refugiums successfully for years.
How loud is a sump system? The main sources of noise are the overflow (water falling from the display into the sump) and the return pump. Modern Herbie-style overflows, which use a full siphon drain rather than a free-fall drain, run nearly silently. A quality DC return pump like the Reef Octopus VarioS adds minimal noise. The protein skimmer pump and air intake can create bubbling sounds. Located in a cabinet with the doors closed, a well-set-up sump system is quieter than most hang-on-back filter setups on a display tank.