Saltwater Sump Tanks

A saltwater sump tank is a secondary aquarium that sits below your main display tank, connected by plumbing. Water drains from the display into the sump, passes through equipment and filtration media inside the sump, and is returned to the display by a pump. Saltwater sump tanks serve as the engine room of a marine system: they house your protein skimmer, heater, refugium, media reactors, and auto top-off equipment while keeping all of that hardware completely out of the display tank.

If you're planning a serious marine or reef system, a sump isn't optional, it's the standard approach that experienced hobbyists use because it works better than any alternative. This guide covers the types of sump tanks available, how to size one for your display, what goes in each section, and the key design decisions that separate systems that run flawlessly from those that flood.

Types of Saltwater Sump Tanks

Sumps come in three basic forms: pre-built purpose-designed sumps, standard aquariums with added baffles, and custom-built acrylic or glass units.

Pre-Built Purpose-Made Sumps

Pre-built sumps are designed specifically for the job. They come with internal baffles already installed, bubble traps to prevent micro-bubbles from entering the return section, and dimensions calculated to fit under standard aquarium stands.

The Eshopps RS-75 is a popular choice for 50-90 gallon display tanks. It measures 24" x 16" x 16" and is designed for tanks up to 75 gallons. It includes a skimmer section, a refugium section, and a return section, with baffles correctly configured for bubble trapping. The 22-gallon capacity provides adequate water volume supplement for most systems in its class.

The Trigger Systems Triton 30 is a step up in build quality, made from cast acrylic with precisely machined baffles. It fits tanks up to 90 gallons and includes a mechanical filter sock holder. At $350-400, it's priced for hobbyists who want something that looks as good as it performs.

The Reef Octopus Classic series sumps are worth mentioning because they're designed to integrate cleanly with Reef Octopus skimmers, making them a natural choice if you're already using that brand's equipment.

Standard Tanks With DIY Baffles

The most cost-effective approach is buying a standard glass aquarium and adding acrylic baffles yourself. A 20-gallon long aquarium (30" x 12" x 12") can be converted to a functional sump for around $30-50 in materials: acrylic sheets ($10-15), aquarium-safe silicone ($5-8), and pipe fittings. For hobbyists comfortable with basic DIY work, this produces a sump that performs identically to a purpose-built unit.

The main disadvantage is setup time. Cutting acrylic to precise measurements, siliconing the baffles, and waiting 24 hours for the silicone to cure takes a few hours of work and planning. Pre-built sumps eliminate that step.

Custom Built Sumps

Custom sumps are built to specific dimensions, typically by local glass or acrylic shops. This makes sense when your stand has an unusual interior dimension, or when you need a specific layout (a larger refugium section, a specialized reactor chamber, or a uniquely deep skimmer section). Custom builds typically run $200-500+ depending on size and whether you use glass or acrylic. Acrylic is lighter, easier to drill and cut for future modifications, and doesn't shatter if dropped, but it scratches more easily than glass.

Sizing Your Sump Tank Correctly

The two sizing questions that matter most are total volume and flood capacity.

Total Volume

A larger sump increases the total water volume in your system. More water volume means slower parameter swings, which translates directly to livestock health and stability. The target is a sump that holds at least 20-30% of your display tank's volume.

For a 75-gallon display, aim for a 20-30 gallon sump minimum. For a 120-gallon display, a 40-50 gallon sump is appropriate. These aren't strict rules, but systems that run with a very small sump relative to the display volume (say, a 10-gallon sump on a 90-gallon display) lose the parameter buffering benefit and may run out of space for necessary equipment.

Flood Capacity

When the return pump stops (power outage, pump failure), water drains back from the display tank into the sump through the return line and the overflow system. This drain-back volume must be contained within the sump without overflowing.

Before your first startup: fill the system and run the return pump. Mark the current water level in the sump. Shut off the return pump and let the system equalize. Measure how much the water level rose in the sump. That is your drain-back volume. The sump must have at least that much additional headspace above the normal operating water level to prevent flooding.

Most 30-40 gallon sumps have 5-10 gallons of available headspace above normal operating level, which is adequate for typical single-drain overflow systems. Very large drain-back volumes (common with wide overflow weirs or high-flow systems) may require a larger sump or a specifically designed drain-back chamber.

Sump Section Layout for Saltwater Systems

Mechanical Filtration Section

Water from the display tank's overflow enters the sump here. A filter sock (typically 100 or 200 micron) installed over the drain inlet catches particulate matter before it reaches the rest of the sump. Filter socks need cleaning every 2-7 days depending on bioload; a neglected filter sock becomes a nutrient factory as trapped debris decomposes.

If manual sock cleaning is something you know you won't keep up with, consider a filter roller system like the Clarisea SK3000 or the Reef Octopus Roller Filter. These advance fresh media automatically and are genuinely worth the $200-300 cost for heavily stocked systems or for hobbyists with unpredictable schedules.

Skimmer Section

The protein skimmer removes dissolved organic compounds before they break down into ammonia and nitrate. Skimmer performance is directly linked to water level stability in this section. Stable water level means consistent skimmer output; varying water level means the skimmer alternates between over-foaming and producing nothing.

Keep the skimmer section large enough that your auto top-off additions don't cause significant level swings. If your ATO adds water in large doses, switch to a more frequent small-dose cycle.

Skimmer sizing for saltwater sump systems: the standard recommendation is to buy a skimmer rated for 2-3x your actual tank volume. A skimmer rated for a "150-gallon system" is often a better match for a 60-75 gallon display than a skimmer rated exactly for your size. Marine systems rarely perform at the "lightly stocked" condition that manufacturer ratings assume.

The Bubble Magus Curve 5, the Reef Octopus Classic 110INT, and the Aqua C Remora Pro are all well-regarded skimmers for the 50-120 gallon sump range at different price points.

Refugium Section

A dedicated refugium section in the sump grows macroalgae (Chaeto, Gracilaria, or Caulerpa) under a dedicated light. As the algae grows, it absorbs phosphate and nitrate from the water. You export these nutrients from the system by harvesting and discarding the algae regularly.

The refugium also provides sanctuary for beneficial micro-crustaceans (copepods, amphipods) that would be consumed in the display tank. Over time, a healthy refugium produces a self-sustaining population of live food for finicky feeders like dragonets.

Light the refugium on reverse photoperiod (on when display lights are off, off when display lights are on) to stabilize daily pH swings. Good refugium lights for small sump sections include the Kessil H80 Fuge ($130), the Tunze 5076 ($75), and the AI Prime Fuge ($150 for the version with a refugium spectrum).

Return Section

The return section holds the return pump and the ATO sensor. Keep this section as clear as possible for pump access. The ATO sensor (float valve or optical sensor) detects when water level drops from evaporation and triggers a fresh RODI water addition.

Evaporation from a saltwater sump system is significant: a 75-gallon display with a 25-gallon sump might evaporate 1-3 gallons per day. An ATO system like the JBJ Auto Top Off ATO, the Tunze Osmolator 3155, or the Neptune Systems ATK kit automates this process and maintains consistent salinity without daily manual intervention.

For a broader look at sump and marine equipment combinations, the best aquarium equipment roundup covers sumps alongside skimmers, pumps, and lighting.

Return Pump Selection for Saltwater Sumps

The return pump moves water from the sump back to the display. Matching pump output to your overflow's rated flow capacity is critical for flood prevention.

Your overflow can only drain a fixed maximum volume per unit time. If the return pump pushes water faster than the overflow can drain it, the display tank overflows. Always match return pump output to the overflow's rated capacity, not to an arbitrary flow goal.

For a standard single-drain 1.5-inch bulkhead overflow, the maximum safe drain flow is approximately 500-600 GPH. A return pump delivering 600 GPH at operating head is the appropriate maximum.

DC variable speed pumps are now standard in the hobby because they let you dial in the exact flow rate needed. The IceCap 1K Pump delivers 1,000 GPH at zero head and around 500-650 GPH at 4-5 feet of operating head, which is appropriate for most residential systems. It runs $80-100. The Reef Octopus VarioS 4 and the Jebao DCP series pumps offer similar variable performance at comparable prices.

For marine systems specifically, use a pump designed for saltwater. Some freshwater pumps use components that corrode in salt environments. Sicce, Eheim, Reef Octopus, and Jebao all make pumps explicitly rated for marine use.

For the top aquarium equipment comparisons including return pumps and skimmers rated for different tank sizes, that resource includes current pricing and specifications.

Common Problems With Saltwater Sump Systems

Micro-bubbles in the display tank. Small air bubbles passing through baffles and entering the return section are a common early problem. Most are solved by ensuring the baffle between the skimmer section and return section is tall enough and has the correct bubble-trap baffle configuration. Raising the baffle height or adding an additional low baffle that water must pass under (not over) usually eliminates bubbles.

Sump overflowing during power outage. Result of insufficient flood capacity. Check by measuring drain-back volume as described above. Solutions: lower the normal operating water level in the sump (reducing it by an inch gives you several extra gallons of headspace), or install a check valve on the return line to reduce drain-back.

Salt creep and evaporation issues. Saltwater exposed to air produces salt creep wherever splashing or misting contacts surfaces. Keep sump openings covered where possible and position your ATO probe to prevent over-addition of fresh water during cycling. Running your sump with a loose-fitting cover reduces evaporation and splash.

Noisy overflow. Water falling from an overhead drain into a lower sump section produces significant noise. A Durso standpipe in the overflow chamber slows the drain fall and dramatically reduces splash noise. A Bean Animal dual-drain system with a proper Herbie configuration is near-silent during normal operation.

FAQ

What size sump do I need for a 75-gallon saltwater tank?

A 20-30 gallon sump is the minimum practical size for a 75-gallon display, with 30 gallons preferred. This gives you enough section space for a skimmer, a small refugium, and an uncluttered return section. If your stand can fit a 40-gallon sump, use it.

Can I use any aquarium as a sump?

Standard glass aquariums work well with DIY acrylic baffles added. Acrylic tanks also work. Avoid using tanks with any previous chemical contamination, and always use aquarium-safe silicone for baffle installation. Confirm the tank isn't leaking before filling it with equipment.

Do I need a refugium in my sump?

A refugium is a significant benefit for reef systems but not strictly required. If you have adequate space in your sump (at least 6-8 gallons for the refugium section), adding Chaeto under a dedicated light provides meaningful nutrient export and stabilizes pH. If space is tight, prioritize the skimmer section and return section over a refugium.

How do I prevent the sump from overflowing if the power goes out?

Calculate drain-back volume before startup (described above). Ensure the sump has at least that much headspace above the normal operating water level. Add a siphon break hole in the return line just below the waterline in the display tank to limit drain-back from the return line. These two steps eliminate almost all overflow risk from power interruptions.

The Core Point

Saltwater sump tanks work because they solve the three main problems with equipment-in-display setups: visual clutter, limited water volume, and inflexible filtration options. Size the sump for your actual flood capacity requirements (not just total volume), design your sections for the equipment you're actually running, and use a return pump matched to your overflow's rated flow. Get those three things right and the sump becomes the most reliable part of your system.