A saltwater aquarium with a sump is a two-tank system where a second, smaller tank (the sump) sits below the display tank in a cabinet, connected by plumbing. Water overflows from the display tank into the sump, gets processed by equipment housed there, and is pumped back up to the display. If you're serious about keeping a saltwater tank long-term, a sump is one of the most important upgrades you can make. It increases total water volume, moves all your equipment out of the display tank, and gives you far more flexibility in how you filter the water.
This guide covers the basics of sump design, what equipment goes in each section, how to size the sump and return pump for your display tank, and the common installation mistakes that cause flooding or equipment failure.
Why a Sump Makes a Saltwater Tank Better
The most immediate benefit of a sump is equipment removal from the display tank. Without a sump, your heater, return pump, skimmer, and any media reactors are all hanging off the walls or sitting in the display tank itself. This looks cluttered, it restricts aquascaping options, and it puts all that electrical equipment in contact with corrosive saltwater in a visible way.
With a sump, all of that hardware lives out of sight in the cabinet. Your display tank contains only rock, sand, fish, and corals. The visual impact is dramatic.
The second major benefit is water volume. A 75-gallon display tank with a 30-gallon sump is actually a 105-gallon system. More water volume means slower parameter swings, which is directly linked to animal health. Saltwater fish and especially corals are sensitive to rapid changes in temperature, salinity, pH, and alkalinity. A larger total water volume buffers against those swings.
The third benefit is refugium potential. If you dedicate one section of your sump to a refugium (a protected area with substrate, macroalgae, and sometimes a separate light), you create a natural nutrient export mechanism and a safe habitat for beneficial microfauna that get consumed in the display tank.
Basic Sump Design: Three-Section Layout
Most saltwater sumps are divided into three compartments by internal baffles.
Section 1: Filter Sock or Mechanical Filtration
Water from the display tank's overflow enters this section first. Mechanical filtration catches debris here before it reaches the rest of the sump. A filter sock (a mesh bag that fits over the drain inlet) is the most common mechanical option. 100-micron filter socks are standard; 200-micron socks clog less frequently but let finer particles through.
Filter socks need rinsing every 2-7 days in a moderately stocked tank. If you don't want the maintenance hassle, a filter roller (like the Reef Octopus Roller Filter) automates the process by advancing fresh mechanical media as it clogs, but these run $200-400.
Section 2: Equipment/Refugium Area
After mechanical filtration, water enters the main sump chamber. This is where your protein skimmer, heater, and any media reactors (phosphate reactor, carbon reactor) live. If you want a refugium, you can add a partition here with a sandbed, Chaeto macroalgae, and a small LED refugium light. The Tunze 5076 Refugium Light and the Kessil H80 Fuge Light are popular options specifically designed for refugium use.
The refugium works best when lit on a reverse photoperiod from your display tank, which stabilizes pH swings. When the display lights are off and pH tends to drop, the refugium lights are on and photosynthesis in the Chaeto drives pH back up.
Section 3: Return Section
The final compartment is where the return pump sits. Water level in this section determines how much goes back to the display. The return pump pulls water from here and pushes it back up to the display via the return line.
This section also needs to hold your auto top-off (ATO) system's sensor. As water evaporates from the display, salinity climbs. The ATO detects the low water level in the return section and adds fresh RODI water automatically to maintain consistent salinity.
Sizing the Sump for Your Display Tank
The sump should be as large as your stand allows. The minimum recommended sump size is 20-25% of display tank volume, but bigger is always better for the reasons described above.
For a 75-gallon display, aim for a 30-gallon sump minimum, 40-gallon ideal. For a 100-gallon display, a 40-gallon sump is minimum, 55-gallon is better.
The critical sizing consideration isn't just volume, it's how much water the sump can hold when the return pump is off. When power goes out or the return pump stops, water from the display tank that's above the waterline in your overflow box will drain into the sump. This "drain-back volume" must not overflow the sump. Run your return pump, note the water level in the sump, then calculate how much of the sump remains empty above that level. That empty space needs to be large enough to hold the drain-back volume.
The drain-back volume for a typical 1-inch overflow is roughly 1-3 gallons. Test it: turn off the return pump and watch how much water drains into the sump. Mark the water level. Make sure you have at least that much additional headroom in your sump.
Return Pump Selection and Plumbing
The return pump moves water from the sump back to the display tank. Return rate is typically calculated as 5-10 times the display tank volume per hour. For a 75-gallon display, you want 375-750 GPH of actual return flow.
The word "actual" matters here. Head pressure from the height the pump must push water, plus friction from the plumbing, reduces effective flow. A pump rated at 1,000 GPH at zero head might deliver only 600 GPH when pushing water 5 feet vertically plus through 6 feet of tubing. Most manufacturers provide head pressure charts.
Popular return pumps for residential saltwater setups:
The Eheim 1262 is a classic internal pump with decades of reliability history. Rated at 900 GPH at zero head, it delivers around 500-600 GPH at 5 feet of head pressure. Quiet, energy-efficient, and very durable.
The Sicce Syncra 4.0 delivers 1,056 GPH zero head and works well as a return pump for 75-125 gallon systems. It runs slightly warmer than the Eheim but is priced attractively at around $70-80.
For larger systems or setups where you want flow control, the Reef Octopus VarioS 4 DC pump is variable speed and controlled via an external dial or controller. Being able to reduce pump speed during feeding or maintenance without fully stopping the sump is genuinely useful.
For a roundup of equipment designed specifically for saltwater setups at different price points, the best aquarium equipment guide covers return pumps, skimmers, and sump designs.
Overflow Systems: How Water Gets from Display to Sump
The overflow is the mechanism that moves water from your display tank to the sump. There are three main types:
Built-in overflow (reef-ready tanks): The cleanest option. The tank has a pre-drilled overflow box built into the back corner. Water overflows the weir, drops through the bulkheads, and travels down plumbing to the sump. No siphons, no hang-on boxes. If you're building a new system, a reef-ready tank is worth the extra cost.
Hang-on overflow box (CPR Aquafuge, Eshopps HOB): Clips onto the rim of a non-drilled tank. Uses a U-tube siphon to transfer water over the rim. These work but have one critical weakness: if the siphon breaks (from a power outage, an air bubble, or debris), water stops flowing to the sump while the return pump keeps running, overflowing the display tank onto the floor. A Durso standpipe or a Bean Animal design on a drilled tank eliminates this failure mode entirely.
DIY single or dual overflow: For hobbyists comfortable with basic plumbing, drilling your own tank and installing a bulkhead is straightforward on glass (not tempered glass) with the right hole saw kit. Two drilled holes give you a two-line "herbie" or Bean Animal overflow system, which is near-silent and virtually impossible to have overflow or flood.
Common Mistakes When Adding a Sump
Undersized sump with insufficient flood capacity. Measure the drain-back volume before your first startup. A sump that's too full during normal operation will overflow when the return pump stops.
Return pump that's too strong for the overflow rating. Your overflow can only drain a maximum volume per minute. If the return pump pushes more water than the overflow can drain, the display tank overflows. Match return flow to the overflow's rated capacity.
Skimmer in a water level that fluctuates too much. Protein skimmers are sensitive to water level changes. If your sump water level rises and falls significantly as your ATO compensates for evaporation, the skimmer output becomes inconsistent. A large return section or a ball valve on your return pump helps stabilize the water level where the skimmer sits.
No check valve on the return line. When the return pump stops, the return line can siphon water backward from the display tank into the sump. A check valve prevents this. Alternatively, drill a small siphon break hole in the return line just below the waterline in the display tank to break the siphon automatically.
For reef-specific sump equipment like protein skimmers and refugium lights, the top aquarium equipment guide covers options across different budget levels.
FAQ
Do I need a sump for a successful saltwater tank?
No, many successful saltwater tanks run without sumps, using only hang-on-back or canister filtration. But a sump makes maintenance easier, provides more stable water parameters, and opens up better filtration options. For reef tanks with corals, a sump is the standard recommendation from experienced reef keepers because of the additional water volume and equipment flexibility.
What size sump do I need for a 75-gallon reef?
A 30-gallon sump is the practical minimum for a 75-gallon display reef. A 40-gallon sump gives you more comfort margin for drain-back volume and a larger refugium section. If your stand can fit a 40-gallon, use it.
Can I use a freshwater tank as a sump?
Yes. Standard glass aquariums without special features work as sumps. You'll need to add your own baffles (typically made from acrylic sheet siliconed in place) to create the compartment layout. Pre-made sumps from brands like Eshopps, Trigger Systems, and Reef Octopus include baffles and are designed to fit under standard aquarium stands, which simplifies setup.
How loud is a sump system?
Noise comes primarily from the overflow water falling into the sump and from the return pump. A Durso standpipe or a Herbie/Bean Animal overflow design dramatically reduces splash noise. A quality return pump like the Eheim 1262 or a DC variable pump runs nearly silently. A well-designed sump system in a closed cabinet is barely audible from a few feet away.
The Core Takeaway
A sump transforms a saltwater aquarium from a cluttered box full of equipment into a clean display backed by a powerful, flexible filtration system. The main work is in planning: sizing the sump for adequate flood capacity, matching the return pump to the overflow's rated flow, and deciding on your sump layout before you drill the first hole or cut the first baffle. Get the plumbing sizing and flood protection right from the start and the rest of the system runs with minimal intervention for years.