Learn what aquarium sumps are used for and how to set one up!

What is a sump?

A sump, in aquarium vernacular, is simply a secondary tank which is plumbed into the primary display aquarium. The purpose of the sump can vary based on it's intended purpose, but typically it is used to hide unsightly equipment such as protein skimmers and heaters. In addition to providing a place to hide equipment it also provides additional water volume to the system. A greater volume of water in the system allows for greater stability.

How a sump works:

In order to have a continuous cycle of water between the display aquarium and the sump, it is necessary to use pump and overflow system. Often new aquariums are advertised as "reef-ready" when they include an overflow box for use with a sump. Overflows allow excess water in the display aquarium to drain into the sump. By constantly pumping water from the sump, the display aquarium will drain from the overflow box and return to the sump where the cycle can start again. In between the drain and pump is where tank equipment can be placed.

The sump is often divided into several sections to prevent bubbles created by the draining water from getting pumped back into the display tank. Although bubbles in the display tank are not necessarily harmful, many people find them unsightly. The various sections of the sump may even feature bubble trap weirs which attempt to keep the small bubbles from traveling across the sump.

The first section of a sump is the drain section. This is where the water from the display tank is draining to. Many often feature a filter medium such as filter floss or carbon pads, these can reduce noise and help filter the water without the use of extra pumps. Often weirs or high walls help trap the bubbles from traveling onwards.

The next section is typically where the sump creator might choose to put filtration equipment such as protein skimmers, heaters or other mechanical filtration. Sometimes these sections can be used for biological media. In the past bio-balls were the preferred medium, but contemporary tanks have often abandoned their use in favor of live rock filtration. Another alternative is the use of a refugium, in which macro-algae is grown to reduce overall nutrients in the aquarium.

The final section is the return section, in which the pump returns drained water from the overflow, back to the display tank. This is the section in which you will see the water level rise and fall depending on evaporation and top off. For this reason it is an unsafe location for most aquarium equipment such as heaters or skimmers which can be damaged when the system runs out of water.

The order of the sections and number greatly varies depending on the needs of the system. If the only need of the sump is to provide additional water volume, then all that is needed is a return and drain section. What are sumps made from?

Sumps are often made by hobbyists from smaller aquariums The main requirement in choosing a tank to use as a sump is determining available size and the largest aquarium which will fit in that space. In certain cases in which stand size is not a limiting factor, any container which can hold water can function as a sump. Often people seeking large tanks for use as a sump will refer to Rubbermaid Livestock tanks. These can be often found at a hardware or farm store.

Sumps sold for retail are typically made out of acrylic with customized sections for the drain, return, and filtration. Acrylic sumps can be fabricated at home with the proper tools, but when purchased at an aquarium vendor they can often be very expensive.

Glass aquariums utilized as sumps won't have any pre-designed sections in them since that was not their original purpose. Section dividers can be easily added though by having glass panels cut at a glass shop or at home. They can be sealed into the aquarium using 100% silicone sealant, often sold at hardware stores and fish stores. It should be noted that acrylic cannot bond with silicone and thus glass dividers or baffles cannot be used in acrylic aquariums, and acrylic dividers or baffles cannot be used in glass tanks.

Pumping water from the sump:

In order for the sump to function properly, an appropriately rated pump must be used in the return section. There are two main factors to consider when choosing a pump for your sump. The first is whether you want an external or internal pump. Internal pumps are placed in the return section submerged in the water. External pumps require the return section to be drilled and plumbed to the pump outside of the actual sump. Often external pumps can be more powerful and prevent heat from being transferred to the aquarium water. If you're located in a hot climate in which your aquarium is at risk of exceeding the safe range (74F to 84F) than you may consider using an external pump. If temperature or size of the pump isn't a factor, then internal pumps can be much easier to plump since there is no need for a drilled hole in the side of the sump.

The second factor in determining a proper pump for the sump is the gallons per hour rating, or how powerful the pump is. The limiting factor in how large of a pump you can use is how fast your overflow can drain water. An overflow box rated for 2000 gph sets your pump limit to 2000gph maximum. Any greater and the overflow will not be able to drain the water fast enough and the display aquarium will overflow. Because the drains could potentially become restricted with algae or snails, it is a good idea to use a pump underrated for the overflow box. The advertised pump rate is fortunately often overrated or measured with no head pressure. The higher your pump must push the water, the greater the head pressure and the slower it will travel. A pump rated at 1200 gph at zero feet of head pressure might only put out 1130 gph at 4 feet of head pressure. Since the pump is often at the bottom of the sump in a stand below the aquarium, a bit of head pressure is usually a factor. Use of a valve on the pumps outlet can help adjust the pump if necessary to prevent the display from filling too fast for the overflow.

Overflow boxes:

Tanks sold as "reef ready" may come with an overflow box built into the tank. These often have holes drilled into the bottom of the aquarium and a standpipe setting the water level inside the actual overflow box. The design of the standpipe can affect the drain rate and noise level. A common overflow box assembly is the Mega-Flow kit, which uses a Durso style overflow standpipe. These standpipes are designed to prevent the pipe from siphoning water out of the overflow and then making a flushing sound when the water runs out. This can be very noisy which is why they are designed to break the siphon by having a hole in the top of the tubing.

Alternative overflows are often drilled in the back panel of the tank rather than the bottom When hobbyists build their own overflows this is often the location of choice because the bottoms of tanks are often tempered glass which cannot be cut or drilled. Having the overflow on the back panel of the tank also prevents there from being as much space taken by the overflow box and less water pressure applied to the bulkhead (the plastic fixture which connects plumbing to the hole in the tank).

If you're adding a sump to a running aquarium your only choice will likely be to install a hang-on-back overflow box which utilizes a siphon to move water from the tank to a secondary box hanging on the back of the tank. This secondary box is where the drain tubing is connected and eliminates the need for a hole to be drilled in the tank. You cannot safely drill a hole in a glass aquarium with any water in it. These siphon overflows can work very well, however if the siphon breaks from air pocket, there is a risk of flooding when the return pump continues to function despite the overflow not properly draining. If a hang-on-back siphon overflow box is used, a float switch should be affixed to the display aquarium to turn off the return pipe if the water gets too high.

Overflow boxes often have teeth cut into the top of them. This allows water into the overflow box but prevents fish or large debris from making their way down the overflow box and to the sump. Covering the overflow boxes can prevent fish from jumping inside in the overflow and getting trapped.

Do I need a sump?

Although sumps are very popular with hobbyists they are not necessary to have a beautiful aquarium. Often beginners are scared away from sumps by their seemingly complex design and potential cost, however if the hobbyist decides to get a large skimmer or wishes to grow macro-algae to reduce nutrients they will be limited to only hang-on back accessories without a sump. The nature of the hang on back accessories can limit their performance and often are only rated for smaller aquariums. While it may be easier to use hang-on-back equipment with a smaller tank, such as those under 50 gallons, larger tanks may be difficult to find hang on-back accessories for.