A reef tank with a sump is a system where the display tank drains into a secondary tank (the sump) below it, which houses your filtration equipment, and then a return pump pushes clean water back up to the display. Virtually every serious reef tank runs on a sump. The reason is straightforward: a sump moves all your equipment out of the display, significantly increases total water volume for parameter stability, and creates space for a refugium that naturally controls nutrients without chemical dosing.

If you're setting up your first reef tank or upgrading from a simple all-in-one system, understanding sump design is the single most important piece of planning you'll do. This guide covers how to design a reef sump, what equipment belongs in each section, how to handle the critical plumbing questions, and what can go wrong if you skip planning steps.

What Makes a Reef Sump Different From a Freshwater Sump

In a freshwater tank, a sump is primarily a space for filtration equipment and extra water volume. In a reef system, the sump serves several functions that are specific to reef chemistry and biology.

Protein skimming: Reef tanks depend on protein skimmers to export dissolved organic compounds before they break down into ammonia and nitrate. Skimmers work by creating fine bubbles that organic molecules attach to, then collecting the resulting foam in a collection cup. Protein skimmers work most effectively with stable, consistent water levels, which a dedicated sump chamber provides.

Refugium: A section of the sump dedicated to macro algae growth (typically Chaeto or Gracilaria) under a dedicated light exports phosphate and nitrate naturally. The macroalgae takes up nutrients as it grows, and you remove nutrients from the system when you trim the algae. This is how high-end reef tanks achieve near-zero nutrient levels without heavy reliance on chemical media.

Calcium and alkalinity reactors: SPS coral reefs consume enormous amounts of calcium and alkalinity as corals build their skeletons. Calcium reactors (which dissolve calcium carbonate media using CO2-acidified water) and dosing pumps typically live in the sump, which keeps the dosing lines short and the equipment hidden.

Temperature stability: A 50-gallon display reef with a 25-gallon sump is actually a 75-gallon system. A 2°F swing in the display is buffered by the sump volume, reducing the actual swing in the total water body. For Acropora and other temperature-sensitive SPS corals, this matters.

Designing a Reef Sump: Layout and Sections

Most reef sumps are divided into three sections using baffles made from acrylic or glass siliconed into the sump walls.

First Section: Mechanical Filtration

Water from the display tank's overflow drains here first. The standard approach is a filter sock (200-micron is popular for balancing fine particle capture against maintenance frequency) that sits over the drain inlet and catches large debris.

An alternative gaining popularity in reef circles is a filter roller or automatic filter roll system. The Reef Octopus Roller Filter and the Clarisea SK3000 use rolls of filter media that advance automatically when clogged, which eliminates the need to manually clean filter socks every 2-4 days. These are legitimate time-savers in high-bioload reef tanks but cost $200-350 upfront.

Second Section: Skimmer Chamber

The protein skimmer needs stable, consistent water level to function correctly. This section is typically sized to keep water level variation below 1/2 inch despite evaporation. A larger section helps buffer level changes between ATO top-offs.

Popular skimmers for the 75-150 gallon reef sump range:

The Bubble Magus Curve 5 handles tanks up to 140 gallons and fits easily in most sump skimmer sections. At around $130-150, it's the entry-level workhorse of the reef hobby. It needs about 6-7 inches of water depth to operate correctly.

The Reef Octopus Classic 110INT is a step up in build quality and performance, handling 110 gallons with a cone-shaped body that produces very fine bubbles. It runs $200-250 and fits sump depths of 6-8 inches.

The Nyos Quantum 120 is a German-designed unit with a self-cleaning mechanism on the neck that reduces maintenance skipping. It handles up to 300 gallons and sits in the $350-400 range for larger reef systems.

If your sump is large enough, dedicate one section to a refugium. A 10-12 gallon refugium section in a 30-gallon sump is enough to grow significant quantities of Chaeto. The refugium section should be separated from the skimmer section by a baffle tall enough that the water level in the refugium is slightly higher than in the skimmer section, which prevents raw sump water from entering the refugium directly.

Light the refugium on a reverse photoperiod. When the display lights are on (daytime), the refugium light is off. When the display lights are off (night), the refugium light is on. This reverse cycle causes the Chaeto to consume CO2 and produce oxygen during the night phase when the display's pH naturally drops, stabilizing your daily pH swing.

Fourth Section: Return Section

The final compartment holds the return pump and the ATO sensor. This is typically kept as open as possible to allow easy pump access and to accommodate the water level variation from top-off additions.

Overflow Systems for Reef Tanks

How water gets from the display to the sump is a critical design decision that affects both flood safety and noise.

Reef-Ready Tanks with Built-In Overflows

The easiest and most reliable option. Manufacturers like Innovative Marine, Red Sea, and Waterbox sell tanks with pre-built overflow boxes and either single or dual drain holes. The Red Sea Reefer series is particularly popular because it uses a dual-drain Herbie overflow design that provides virtually silent operation and automatic flood protection.

A Herbie overflow uses two drains: a full siphon drain that runs continuously at the exact return flow rate, and an emergency drain that's normally dry. If the main drain gets blocked, the emergency drain handles the flow and prevents the display from overflowing. The sound of water falling from the emergency drain tells you immediately that something is wrong.

Hang-On Box Overflows

For tanks that aren't reef-ready (pre-drilled), a hang-on overflow box uses a U-tube siphon to move water over the rim. These work but carry an inherent risk: if the siphon breaks, the overflow stops flowing while the return pump continues, overflowing the display tank onto the floor.

If you must use a hang-on overflow on a reef tank, use one with a built-in siphon maintenance system. The Lifereef VFO Series uses a standpipe design that helps maintain the siphon. The CPR CS50 is a popular entry-level option. Add an AquaLifter pump to the U-tube to continuously remove air bubbles, which dramatically reduces the likelihood of siphon failure.

Drilling the tank is almost always preferable to hang-on overflow if you can do it. Most non-tempered glass tanks (and virtually all modern reef tanks sold through aquarium retailers) can be drilled with a diamond hole saw kit for under $30.

Return Pump Selection

The return pump needs to move enough water to refresh the display volume several times per hour while not exceeding the overflow's rated drainage capacity. For reef tanks, the typical target is 3-5 times total system volume per hour for the return pump specifically (additional flow comes from powerheads and wavemakers in the display).

For a 50-gallon display reef with a 20-gallon sump (70 gallons total), a return pump delivering 300-400 GPH at operating head pressure is appropriate.

DC variable speed pumps have become standard in the reef hobby because you can dial in the exact flow you need rather than over-plumbing and throttling with a ball valve. The Reef Octopus VarioS series, the IceCap 1K and 2K return pumps, and the Varios pumps from Jebao are all popular in the $80-200 range.

For matching your sump design with a complete equipment list, the best UV sterilizer for reef tank guide covers one of the common additions to a mature reef sump setup.

Water Quality in a Sump-Based Reef

One of the most important inputs for a sump-based reef system is the quality of your top-off and makeup water. As saltwater evaporates from the display, only pure water leaves (salt and other dissolved minerals remain). Replacing that evaporation with tap water introduces silicates, chloramines, phosphates, and other contaminants that accumulate over time and drive algae growth and parameter instability.

Every sump-based reef should be paired with a quality RODI system for makeup water. A system like the BRS 4-Stage RODI or the Spectrapure 90 GPD removes dissolved solids to near-zero TDS, giving you a clean baseline for both water changes and top-off. For a comparison of options, the best RODI unit for reef tank roundup is the right starting point if you haven't set up RODI yet.

Common Sump Mistakes in Reef Systems

Sump too small for drain-back volume. When the return pump stops, water drains back from the display through the return line and from the overflow box. This drain-back volume must fit in the available headspace in the sump. Measure it before your first startup by shutting off the return pump and watching where the sump water level settles.

Skimmer positioned where water level varies too much. If your ATO tops off several gallons at a time in the skimmer section, the skimmer level changes and the output becomes unpredictable. Keep ATO additions small and frequent rather than large and infrequent, and size the skimmer section large enough to buffer minor level changes.

No siphon break on the return line. A small hole drilled into the return line just below the display waterline prevents the return from siphoning backward when the pump stops. Without it, you can lose several gallons of display water into the sump on every power interruption.

Chaeto refugium without reverse photoperiod. Lighting the refugium on the same schedule as the display tank means both are consuming CO2 during the day and both are producing CO2 at night, which amplifies pH swings. Reverse photoperiod is a free way to stabilize your daily pH variation by up to 0.1-0.2 pH units.

FAQ

What size sump should I get for a 75-gallon reef?

Use the largest sump that fits in your stand. A 30-gallon sump is the practical minimum; a 40-gallon is better. The standard 30-gallon aquarium (36" x 12" x 16") fits under most 75-gallon stands and provides enough space for a three-section design with a reasonable refugium.

Do I need a protein skimmer if I have a refugium?

Both are most effective when used together. A refugium exports nutrients via macroalgae growth, which is slow but consistent. A skimmer exports dissolved organics before they fully break down, which is faster but requires maintenance. Heavy reef tanks with large coral collections benefit from both running simultaneously.

How much does it cost to set up a sump for a reef tank?

A complete sump setup for a 75-gallon reef, including sump tank, baffles, protein skimmer, return pump, ATO system, and return plumbing, runs $400-800 depending on brands selected. The sump tank itself can be a repurposed glass aquarium (add your own acrylic baffles for $20-30 in materials) or a pre-designed sump from Eshopps, Trigger Systems, or Reef Octopus for $120-300.

Can I add a sump to an existing reef tank that's already set up?

Yes, but it requires careful planning. You'll need to either drill the existing display tank for a drain (if it's drillable glass), add a hang-on overflow box, or restart with a new drilled tank. The sump itself can be added without disrupting the existing tank once the plumbing connections are in place. Many hobbyists add a sump during their first major tank maintenance when they'd be doing a partial teardown anyway.

The Core Point

A sump is the foundation of a high-performing reef tank, not an optional extra. Get the design right at the start: a sump large enough to handle drain-back volume, a reliable overflow system (drilled tank is best), a protein skimmer sized for your bioload, and a refugium if space allows. These investments pay back in cleaner water, more stable parameters, and a display that looks exactly like what you imagined when you started.