External canister filters are the filter of choice for aquariums over three feet in length and combine mechanical, biological, and chemical filtration to clear water and break down waste.

They’re termed “external” because the pump and canister sit outside the aquarium, usually in a cabinet underneath, and the filter is connected to the tank via an inlet hose and an outlet hose. 

Standard internal canister filters need to be small and discrete so they don’t take up too much room in the aquarium, but externals can be much larger (typically 7 litres in volume or more,) meaning that they are much better equipped at removing and breaking down much larger amounts of fish waste. This makes them particularly suitable for heavily stocked aquariums like Malawi cichlid tanks, messy fish like Oscars, Plecos or Goldfish, or any tank where the clearest, cleanest freshwater is important. 

External aquarium filters come in different sizes so choose the model which is suited to your aquarium’s maximum volume, like 300 litres for example, but extreme circumstances may mean that you fit two external filters to one tank (with one at each end,) or you fit the largest filter model available. The bigger the filter the more powerful the pump, so external filters can also create strong water flow in the aquarium, perfect for fish species that come from fast-flowing rivers.

You can also pack external fish tank filters with your own bespoke filter media, so you can change the biological media to the brand of your preference or add different media like peat, phosphate remover or polishing pads.  

How to set up an external filter

Open up the canister filter and place the media. Some filters come with media and some without, so check first. Most external filters come with baskets to house different types of media so mechanical filter media like sponges and filter floss for mechanical filtration should be placed where the dirty water enters the filter, then the biological media, for beneficial bacteria, then the chemical media like carbon. Carbon may need to be rinsed before use to remove any fine dust. Ensure the sealing rings are in place, clip the filter shut and place the canister in the cabinet underneath the tank. 

At the tank end, assemble the inlet and outlet. The inlet will have an inlet strainer to prevent fish from being sucked in and the outlet may have a spray bar return option. Use hot water to supple up the PVC hose then slide it over the hosetail on the inlet and outlet assemblies. Do up the locking nuts to secure the hose and clip the inlet and outlet in place with the supplied rubber suckers.

Next, run the hoses down the back of the tank and into the cabinet. Meet them up with the top of the filter, give yourself another few inches of hose just to make sure, and then cut them to length with sharp scissors. Dip in hot water before sliding the hoses onto the top of the filter and do up the locking nuts to prevent leaks. The filter should say which hose is IN and OUT.

Which is the best external filter?

All external filters are good and all work in a similar way. If you want value look for a filter which comes with all the media you need. If you want power, opt for a model which produces over 1000lph. Some extra-large models produce over 2000lph, making them suitable for aquariums over 1000 litres in volume, but in general, if you are in the market for an external filter, choose a large one. 

Some external filters come with a heater built-in, meaning that you won’t need a separate one in the main tank. This offers two big advantages - the heat is evenly distributed around fish tanks by the filter outlet and the heater is tucked away where it can’t be damaged by large, destructive fish or cause burns to small catfish which like to hide under them. You do pay a premium for a filter with a heating element but it’s a useful feature if your budget allows.

If you want state of the art, choose a model with app control. You’ll be able to control the output on your phone and it’ll tell you when it’s starting to get blocked and flow is restricted. 

While you're thinking about your aquarium filter, you might want to stock up on aquarium filter media. Why not try our brand-new pro filter media including Swell Aquarium Activated Carbon, Swell Aquarium Nitrate Remover Media, Swell Aquarium Phosphate Remover Media, Swell Aquarium Ultra Porous Biological Filter Media and Swell Aquarium Nitrate Remover Resin?