How to choose an Aquarium Filter

The filter is the most important piece of equipment for any aquarium, but there is a huge array of models available, so how do you choose? Here’s our essential guide.

The Basics

All aquariums containing fish need a filter of some kind. The filter traps debris, clearing the water and enabling better viewing of your fish. The trapping of physical waste is called mechanical filtration.

Fish produce an invisible waste product as they breathe and defecate, called ammonia. And although its produced by fish, it is very toxic to them, and in nature it is diluted by the vastness of the body of water in which the fish live, like a lake, river, or ocean.

In the aquarium ammonia needs to be converted into less toxic substances by friendly bacteria. Conversion of waste by bacteria is called biological filtration, and without it, aquarium fish will poison themselves, and die.

Water can also be further purified by exposing it to special porous materials. Activated carbon and resins can adsorb some substances from water like dyes, odours, chlorine, nitrate or phosphate. This is called chemical filtration.

So biological filtration is essential to keep all fish alive in aquariums. Mechanical filtration keeps water clear, and chemical filtration can be used to further clean water if desired. A good aquarium filter will enable mechanical, biological and chemical filtration, and will keep your fish alive and your aquarium healthy.

Internal or External?

Aquarium filters are available in two main types – internal and external. Internal canister filters should be placed inside the aquarium, and are inexpensive, easy to fit, and simple to maintain. They consist of a pump, to draw dirty water into the filter, and a canister, where the media is placed.

At its most basic an internal filter will consist just of a pump, canister, and a sponge, with the sponge filtering both mechanically and biologically. The sponge will become clogged over time, so should be cleaned in old tank water, never washed under the tap, (see below) and periodically, a portion of sponge should be replaced. Filters with two sponges instead of one, enable alternate cleaning.

The best internal filters will have three separate areas for mechanical, biological and chemical filtration. With separate, dedicated biological filter media, mechanical filters like sponge or floss can be cleaned or replaced more regularly, without damaging the all-important beneficial bacteria. Activated carbon or carbon impregnated pads occupy the chemical filtration area, giving the water a final polish before being returned to the tank.


A good internal filter will also come with a venturi – a special nozzle on the pump return pipe which draws in air and shoots fine bubbles into the water. Venturis are beneficial for all fish as they also do the job that an air pump and air pump would do, further agitating the water surface, and increasing dissolved oxygen levels. Oxygen is good for fish and filter bacteria, and venturis can always be turned off if desired.

External Filters

External filters are designed to be placed in the cabinet underneath an aquarium, and connect to the main tank using an inlet pipe and an outlet pipe. Water is drawn down from the main tank by a pump on top of the filter body, and is then exposed to a combination of mechanical, biological and chemical filter media.

The advantage with external power filters versus internals is that they have a much larger filter capacity, and usually a stronger pump. This makes them more suited to large aquariums, or aquariums with heavy fish stocking levels, or messy fish like cichlids or goldfish. They take a bit more to set up because the pipework must be cut to length, positioned and properly secured, but are versatile in that other media types can be used, and other outlet options, like spraybars.

Internal Filters


Small, easy to fit, inexpensive to buy and maintain. Great for beginners.


The small canisters make them only suitable for tanks of 120cm/48” or less.

External Filters


Large, powerful, and suitable for large and very large tanks. Versatile media options.


More expensive to buy versus internal filters, and more daunting for first time fishkeepers

Top Tips!

Don’t wash that sponge under the tap!

A very common mistake with new fishkeepers is to wash the filter sponge under the tap, to clean it. Chlorine and Chloramine in tap water damages and destroys filter bacteria, leaving your filter incapable of converting toxic fish waste. Ammonia will quickly build up, making fish sick, and killing them.

Always wash filter sponges in a bucket of old tank water, removed during a water change. Never replace all old sponges with new, as the same thing will happen. The biological filter will be brand new and sterile, and incapable of converting toxic ammonia. 


Can I put my heater in my external filter?

Yes, some models come with heating built-in, negating the need to have a conventional heater/thermostat in the main tank. They’re known as thermo filters, and benefits include even heat distribution as pre-warmed water is pumped around the tank, and no unsightly heaters on show in the main tank, spoiling the vista. They are good with big, destructive fish too, as the heating element is safe in the filter underneath the tank, where it can’t be damaged.

If you already have an external filter and want to make it into a thermo, you can, with an inline heater. The same advantages apply, and they can be retro-fitted to most external filters.

Is there anything I can do to replace beneficial bacteria lost during cleaning?

Yes, beneficial filter bacteria is available to start-up biological filters in newly set-up tanks, and to replace bacteria lost during cleaning. Arm yourself with it if its a new tank, or water test results reveal levels of ammonia or nitrite.

Should I turn the filter off at night?

No, never turn a filter off at night, as the beneficial bacteria you are trying to encourage will die due to lack of oxygen and water flow. You’ll risk your fish’s lives too, as they also need oxygen provided by filter, and surface agitation.                  

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Jeremy Gay is an author, lifelong fishkeeper, and aquatic specialist. He's a former editor of Practical Fishkeeping Magazine, UK editor at Reefbuilders, a former aquatic store manager, and has collected fish in Sri Lanka and the Amazon. He's been on tv and radio, contributed to Koi Carp and Gardeners World magazines, been a product tester, a judge, and a product developer. Jeremy is here to guide and advise you on all things tropical, pond and marine, from set-up to stocking, health, feeding to breeding, as well as solving many common fishkeeping problems along the way.

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