A marine fish tank with a sump is a system where a secondary tank or container (the sump) sits below the main display tank, connected by overflow plumbing so water flows down by gravity and gets pumped back up by a return pump. This setup gives you dramatically more water volume, a hidden place for equipment, and better water quality stability than a stand-alone tank.
Most serious reef keepers eventually migrate to a sump-based system, and for good reason. Sumps let you keep heaters, skimmers, reactors, and return pumps out of the display tank where they clutter the view. They add 20 to 50 gallons of water volume to the system (more water volume means more stable parameters). And they give you a dedicated area for mechanical filtration where you can swap filter socks without disturbing the main tank. If you are starting a marine tank and wondering whether to invest in a sump from the beginning, the answer is almost always yes.
How a Marine Tank Sump System Works
Water flows from the display tank down into the sump through one of two mechanisms: a drilled overflow with a bulkhead fitting, or a hang-on-back overflow box. From there, gravity does the work.
Drilled Overflows and Bulkheads
A drilled overflow creates a hole in the side or bottom of the tank, through which a bulkhead fitting is installed. Water rises to the level of the overflow opening and spills into a drain pipe that leads down to the sump. Drilled overflows are more reliable and quieter than hang-on-back boxes. Most modern reef-ready tanks like the Red Sea Peninsula or Innovative Marine Nuvo series come pre-drilled with overflow chambers built in.
The Herbie drain method uses two drain lines from a single overflow: one as a full siphon for quiet drainage, and one as an emergency drain that sits higher up and catches flow if the primary siphon breaks. This is the most reliable approach for managing drain noise and preventing sump overflow if the siphon fails unexpectedly.
Hang-On-Back Overflow Boxes
If your tank is not drilled, a CPR Aquafuge overflow box or similar device siphons water over the rim. These work but carry a significant risk: if power goes out, the siphon can break when power returns and cause the sump to overflow onto the floor. A battery-powered air line connected to the siphon box helps maintain the siphon during outages.
The Return Pump
The return pump sits in the sump and pushes water back up to the display tank. Sizing matters. You want to turn the sump's total water volume over 3 to 5 times per hour. For a 30-gallon sump, that means a pump rated 90 to 150 GPH at your actual head pressure (accounting for pipe length and height). The Eheim Compact 1000+, Reef Octopus VarioS series, and Neptune DOS pump are popular choices because they are quiet and energy efficient.
Always size the return pump so it can be throttled down. Running a pump at 80% capacity extends its life and makes it easier to dial in flow without the tank overflowing or the sump running dry.
What Goes in the Sump
The sump layout typically flows from display tank to equipment area to return section.
Skimmer Section
The protein skimmer sits in the first section of the sump where water enters from the display tank. Skimmers work best at specific water depths (usually 6 to 10 inches), so check your skimmer's recommended water level before finalizing sump chamber dimensions. The Reef Octopus Classic 110-INT and Bubble Magus Curve B-7 are commonly used in mid-size reef systems (75 to 150 gallons).
Refugium Section
A refugium is an optional middle compartment where you grow macroalgae like Chaeto (Chaetomorpha) under a reverse-cycle light (the refugium light is on when the main lights are off). Chaeto absorbs nitrates and phosphates as it grows, acting as a natural nutrient export. You harvest and discard a handful every week or two, and with it goes the nutrients the algae absorbed. The Kessil H380 Refugium light is purpose-built for this application.
Return Section
The final section holds the return pump. This is also where you place your auto top-off (ATO) sensor and any dosing pumps. Keeping these devices in the return section prevents them from interfering with skimmer performance.
Building or Buying a Sump
Ready-made sumps like the Trigger Systems Triton 39 or the Simplicity Aquatics 30-gallon sump come with pre-built chambers, filter sock holders, and clearly marked return sections. They run $150 to $400 and save significant time compared to DIY.
DIY sumps built from standard 20-gallon long or 40-gallon breeder tanks with silicone-bonded acrylic baffles are a popular option for budget-conscious setups. A custom 40-gallon breeder sump with three baffles costs about $60 in materials plus the tank (often available used for $25 to $40 on Craigslist or local hobbyist groups).
For sourcing equipment and sump components, the Best Online Fish Supply Store roundup lists retailers who carry full sump kits and individual components at competitive prices.
Plumbing Materials and Best Practices
PVC vs. Flexible Tubing
Schedule 40 PVC pipe is the standard for fixed drain and return runs. It is rigid, smooth, and resistant to algae buildup. Use flexible spa flex tubing or Loc-Line modular hose for the last 12 to 18 inches before the display tank return nozzle so you can aim flow easily.
Never use copper pipe or brass fittings in saltwater plumbing. Copper is toxic to invertebrates at low concentrations.
Gate Valves
Install a gate valve (not a ball valve) on the return line to fine-tune flow. Gate valves allow precise adjustment. Ball valves are either fully open or closed, making precise flow control difficult.
Handling Power Outages
When power goes out, the return pump stops and water continues flowing into the sump from the drain until the display tank drops to the overflow level. Your sump must be able to hold this "back-siphon" volume without overflowing. To calculate it, mark the water level in the sump when the system is running, then turn off the return pump and watch where the water level stabilizes. That difference is your back-siphon volume. Always build 15% extra capacity beyond that measurement into your sump size.
For oxygenation during power outages, a battery-powered air pump connected to a bubbler stone keeps water oxygenated until power returns. For options on aeration equipment, see the Best Oxygen Machine for Fish Tank Price guide.
FAQ
How big should a sump be for a marine tank? A good general rule is 20% to 30% of the display tank volume. For a 100-gallon display, a 20 to 30-gallon sump is a practical minimum. Bigger is better for stability. A 40-gallon sump on a 100-gallon display adds substantial buffer against parameter swings.
Can I run a reef tank without a sump? Yes, especially on smaller tanks under 50 gallons. Many all-in-one tanks like the Red Sea Max E-170 or Innovative Marine Nuvo 30L use rear filter chambers that function like a mini-sump. But for larger systems and more serious reefing, a dedicated sump is worth the plumbing investment.
Does a sump eliminate the need for other filtration? Not completely. A sump with a protein skimmer and refugium handles biological and nutrient export effectively. But you still need live rock for beneficial bacteria colonization, and you may need a reactors (like a calcium reactor or GFO reactor) depending on your coral load.
How do I stop the sump from making noise? Drain noise usually comes from turbulence in the drain pipe. The Herbie drain method (full siphon primary drain plus raised emergency secondary) virtually eliminates gurgling. For return pump noise, mount the pump on a foam pad to isolate vibration and use flexible tubing for the last section of the return line.
A sump is not an optional upgrade for a serious marine system; it is the infrastructure that makes long-term success practical. Getting the plumbing, sizing, and section layout right from the start means fewer issues down the road and a much simpler maintenance routine.