What to Know About Aquarium Sump and Overflow Box Setups

Little fish in fish tank or aquarium, gold fish, guppy and red fish, fancy carp with green plant, underwater life.
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As an aquarium owner, you have to make a lot of choices to keep your tank and fish safe.

The sump reservoir is ideal for aquarium owners who want to keep their filtration equipment out of the main aquarium's space. Sumps are a staple of most marine and reef tanks, but they can be used on freshwater tanks, too. This extra reservoir can increase the aesthetics of your tank and allow for maintenance to occur without disturbing the tank's occupants.

What Is a Sump Filter?

A "sump" refers to an external water reservoir below the aquarium that water flows into before being pumped back up into the main tank. The sump usually holds additional filtration components.

Sumps are typically made of glass or acrylic, but some aquarium owners also get creative by using storage tubs or old, unused tanks to make sump reservoirs. Most sumps will sit under your tank, so before you get started, make sure you have reasonable space and access.

  • Ease of maintenance to socks and other biomedia

  • Increase your water volume to improve water quality

  • Ability to add other filter components more easily

  • Increased tank aesthetic

  • Can overflow and drain tank if not set up properly

  • Decreased available storage space for tank accessories (nets, décor, treatments, etc)

  • Slight increase in noise

Other Types of Filtration

If sumps seem too ambitious, there are two other filter methods you should research.

Hang-On Filters

This type of filter is simple and straightforward. Simply hang the filter on the back of your tank, prime it by adding water to the reservoir, and plug it in. What's recommended is using sturdy sponges as compared to flimsy floss. These are critical to the building of your biofilter and should never be replaced.

Canister Filters

These filters are external, like a sump, but are protected from overflows and accidental drainings by being pressurized. Similar to a sump, you will set up the intake and return tubes from your main tank to the filter that usually sits under your tank. Inside the canister is a variety of mechanical (link) and biological (link) filters. As with the hang-on filters, it is critical to your biological filtration that you never replace these components.

Both of these options are easy alternatives to a sump. However, for the reasons listed above, sumps can be a great addition to your tank if you’re looking for increased filtration capabilities.

What Do You Need to Set Up a Sump?

Most commonly, a sump setup will be sold as a kit with biomedia. You can use this or a simple plain aquarium and add your own media. Sumps are filled using gravity with a pump to return water to the tank. You will need appropriate hoses to move the water back and forth from your tank. Employ a skimmer box to cycle water in between your tank and sump. This will decrease the pull of the outflow to prevent animals from being sucked into the sump. It also acts as a fail-safe to protect your sump from accidentally draining your tank.

Skimmer box and overflow
Jessie Sanders

How to Correctly Set Up a Sump

  1. Place your overflow skimmer that will supply your sump in an area that is away from decor, feeding zones, or any fish hiding places. Ideally, you should set up your outflow and inflow in opposing spots so it creates a gyre, or system of currents, in your tank.

  2. Connect tubing between overflow skimmer to sump inflow. If you are making your own sump without a kit, inflow from the tank should come into one side of the tank and return on the opposite side. Ideally, your inflow should cover the top of the tank and the outflow collects water from the bottom after it flows over the biological filter media you have chosen.

  3. Set up your return hoses from the sump pump to return water from the sump to the tank. If you lose power, your sump will have to hold the extra water in the lines and any additional water that is pulled via gravity.


    Keep the return hose close to the top of your water line in your main tank. If you lose power or your pump fails, this way it will not drain your tank. This is also why you should place air pumps above your tank.

  4. Choose a sump size and fill line to accommodate any accidents.

  5. Position the return so it flows through the tank before reaching the outflow. If your outflow is too close to the return to your sump, you are not filtering the entire volume of your tank.

    Sumps usually pull water from your tank via gravity and return it by using a “sump” pump. This is not the same type of sump pump that keeps your basement dry. Most kits will have a pump appropriately sized to match the flow of gravity from the skimmer basket and tubing size selected. If you are making your own, it is critical to match the flow of gravity to the pump return.

  6. To gage the outflow, start your skimmer basket by priming it with water from the main tank. Catch the outflowing water in a measured container. Once the flow has established, time 10 seconds and watch to see how much water flows out. The gallons per second will tell you exactly what type of pump to buy. If you are unable to perform this test, buy a pump with a variable speed switch to manipulate the flow to match the outflow.

  7. With the correct size pump, start the gravity outflow and plug your pump in. Watch your sump line carefully to make sure it isn't creeping up or sinking too low. You may have to manipulate your pump setting to match the flow of gravity.

  8. Check your sump level a few times over the next 48 hours and make adjustments as necessary.

As with all other biomedia, it will take approximately four to six weeks for the media to be fully "cycled."

When done correctly, sumps can be a great tool for your aquarium. They can increase your filtration capacity, increase your total water volume for improved water quality, and create a better environment for your fish.