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Sump filters

When do I need one

Sump filters are very large filters, capable of handling high volumes of water and processing it quickly. They are only practical on large water systems, such as display tanks in shops, multiple fish tanks in a single location or a large aquarium in the home.

If you have 100 gallons (~454 litres, 120 US gallons) or more in one tank, or a combination of tanks in a rack, then a sump is worthwhile, as long as you are able to connect the tanks to it. This usually involves drilling the glass to take piping, so, on an established setup, it isn’t always possible. I am considering installing a sump on my living room display, because there are 6 tanks, all on the same level around two walls, but to do this I have to take each tank down, drill a hole in the bottom to take the pipe connector and then set the tank back up. I also have to install piping to take the outlet flow from the tank to the sump, and piping to put the filtered water back into the tank at the top.

So why bother?

6 tanks all need regular water changes and conditions change more quickly in each tank, all the tanks require their own filters and heaters to maintain the conditions, and separate air supply for ornaments etc. By linking all these tanks into a sump filter it will make water changes easier, as there is only one system to change, water conditions will remain more stable throughout all the tanks and heating will be more efficient. Also, the inlet from the sump will create a strong current in the tanks, something that the UGF doesn’t do.

I am fortunate in as much as when I built the shelf to take the tanks, I planned ahead a little and incorporated hidden areas where I could run piping if I ever decided to make the change. So, for me, it’s not such a major conversion, but for other setups, it may not be so easy.

What are the advantages of a sump over a canister or HOB filter?

  1. Volume:

    Canister/HOB filters can only handle a set volume of water. A sump is only restricted by the size of the piping you install and the capacity of the pump you use.

  2. Flexibility:

    A sump is completely flexible as to the media you use. It can range from gravel to beads, ceramics, floss, foam or almost any media you choose. It can also take activated carbon, heaters and other accessories, depending on the size you make it. The only caution here is that whatever media you use doesn’t restrict the flow too much!

  3. Efficiency:

    Because you can control the media, you control the efficiency of the filter.

  4. Stock levels:

    Because the sump holds a volume of water, the tanks connected to it can be stocked above normal limits without overstocking. This is because the volume of water dictates the number of fish more than the size of the tank itself. For example, a 100 gallon (~454 litres, 120 US gallons) tank connected to a 30 gallon (~136 litres, 36 US gallons) sump actually means that the fish have 130 gallons (~590 litres, 156 US gallons) of water, so stock levels can safely be increased by 30% without overstocking.

  5. Stability:

    It is much easier to maintain water quality in a large tank than in a small one. This is because the waste that fish produce is more diluted in a large water system than a small one. So it makes sense to make your system as large as possible.

However, as with anything, there are down sides. Here are some of the ones I have found and anticipate:

  1. Leaks:

    As with all plumbing, it is susceptible to leaks. The more piping you have, the more chance there is of problems with water escaping.

  2. Flooding:

    With more water in the system, a major leak releases a lot of water. Also, unless the sump is correctly set up, a power outage can cause flooding through settling in the tanks above overflowing the sump capacity. You may also have problems with siphoning, if the inlet is below water level during normal operation, so it is worth considering a control valve to avoid this during planning.

  3. Disease:

    Obviously, if a disease gets into the system, it could spread to all the tanks connected to the system and be very expensive to treat. This is why it is important to ensure that water quality remains optimal at all times, and that new fish are quarantined for a period before introduction. It also gives weight to the addition of UV sterilisation being built into the system.

In conclusion:

Sumps have a place in the aquarists toolbox, if the right conditions exist. If you are planning a large aquarium, you may want to consider a sump as an alternative. If you have several existing tanks in the same location, then a sump may be beneficial. If you are planning to build a fish house or breeding rack, then a sump should definitely be worth consideration.

Just remember that some rules apply to the size of sump in comparison to the water system it controls and enough room must be built in to take any settling in the event of power failure. The difference between working and settling levels is also affected by the amount of water flowing from the sump to the tank, as the faster the flow, the higher the working level and the more water has to settle!

Questions and answers

You're welcome to submit your own questions regarding sump filters, however make sure that they're unique and not yet answered on this page, please. Originally we were publishing questions at, but merged them with related articles later. Here are two questions:

What size sump do I need for a 100 gallon tank?

Answer: On my 100 gallon (~454 litres, 120 US gallons) tank I am running a 36” (~91 cm) sump that is 18” (~45 cm) deep. The most important factor is how efficiently that it has been divided into separate compartments for holding the pump, media etc. I made three chambers so that in the second I could add a Deep Sand Bed for the filtration.

What media should I use in a sump?

Answer: In the first chamber, sponges should be added for the mechanical filtration, a Deep Sand Bed is the only other media that needs to be used unless you wish to add phosphate remover.

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