What is a Sump?

A sump is a container of water that sits below your display aquarium that is used to house your filtration and additional life support equipment so you don't have to hang or submerge that gear in your display.  This not only provides more room for more advanced equipment but it is easier to maintain and keeps your display aquarium free from clutter.  You also increase the system's total water volume which ultimately helps improve stability.

View Diagram: What Is A Sump? - Filtration and Refugiums For Your Aquarium

How Does a Sump Work?

Water drains from the display aquarium down into the sump via a drain in the display.  That water passes through the sump filtration and is then pumped back up into the aquarium via the return pump.  As the water level rises in the display, the water overflows down through the drain again and back into the sump and the cycle continues.  Aquarium water constantly recirculates between the display and the sump. The drain in your display is what keeps the water level constant because any and all water that comes up through the return pump simply forces the same amount of water to flow back down through the drain and into the sump.

View Diagram: How to Plumb a Sump - Plumbing Diagrams For Your Aquarium Sump

What Size Sump is Best?

Most would agree that the largest possible sump that will fit into the space you have is the best choice.  A larger sump means more water volume which means more dilution of waste. It also provides more room for equipment and simplifies some aspects of your maintenance.  Of course, the larger the sump, the larger the investment. 

The general rule is no less than 25% of your tank's total water volume for the best results.  Larger is acceptable but it should not be any smaller.  For example, a 100-gallon display aquarium should have a sump that can hold at least 25 gallons of water. 

What Should You Put in a Sump?

The equipment you choose can vary based on the particular method of filtration or type of reef aquarium you plan to keep. There really is an incredible variety of different pieces of filtration and other life support equipment that would work inside a sump and benefit your aquarium so it would be tough to really define this in a way that applies to everyone. 

With that in mind, here are the typical pieces of equipment you will probably find in 90% of the sump used in saltwater aquariums along with a list of some additional gear that we find to be particularly useful. 

Common Equipment

  • Mechanical filtration - Removes suspended particulate waste and would include filter socks, filter floss, and fleece filter roller. 
  • Protein Skimmer - Removes dissolved organic waste.
  • Refugium or Macroalgae Reactor - Removes nitrate and phosphate from the water and helps to increase biodiversity.
  • Media Reactor - Designed to house chemical filter media such as Carbon and GFO. 
  • Heater - Maintains stable water temperatures.
  • Return Pump - Pumps water back into the display aquarium, often called "the heart" of your aquarium. 

Additional Items

  • Biomedia
  • Probe holder
  • Dosing and ATO tube holder
  • UV Sterilizer

Is a Sump Better Than a Canister Filter?

For reef aquariums, most would agree as a sump is better than a canister filter for a few reasons. 


  • Increased water volume
  • Contains and hides equipment
  • More space
  • The overflow system helps maintain a constant water level with the use of an ATO
  • Serviceable while in operation
  • More versatile

Canister Filter

  • Compact
  • More affordable
  • Easier to install