What size filter do I need for my tank?

Look for a filter rated to turn over several times your tank's total water volume each hour, and check the tank size range stated on the packaging as a starting point. Bigger is not always simply better, since flow rate needs to suit the fish you keep too.

How often should I clean an aquarium filter?

It depends on the filter type and how heavily stocked the tank is, but rinsing media in old tank water periodically rather than replacing it all at once is the general approach, since this avoids disrupting the bacteria colony the filter hosts.

Why does my new filter need time before the tank is ready for fish?

A filter's biological role depends on a colony of bacteria that takes time to establish, a process often called cycling. Adding fish before that colony is established can expose them to waste levels the filter is not yet able to process.

Can a filter be too powerful for my fish?

Yes, particularly for fish that prefer calm water, such as some long-finned or slow-swimming species. A filter's flow rate matters as much as its capacity, so matching it to the fish you keep is worth checking rather than assuming stronger is always better.

Aquarium filter — what to look for

An aquarium filter's job is to keep water clear and safe by removing waste and supporting the bacteria that break it down, and the right one for your tank depends mainly on tank size, the type of fish you keep, and how much flow those fish can tolerate — not on picking the biggest or most powerful option available.

Why filtration matters more than it looks like it should

A fish tank is a closed system, and without filtration, waste from fish, uneaten food and decaying plant matter builds up quickly and becomes genuinely harmful to the fish living in it. A filter does two jobs at once: physically removing debris, and hosting the bacteria that convert toxic waste into less harmful compounds.

That second job — the biological side — is why a new filter needs time to establish before a tank is considered fully cycled, and why replacing filter media entirely rather than in stages can set a tank back by removing the bacteria colony living in it.

The main filter types

Internal filters sit inside the tank and suit smaller setups well, being straightforward to install and maintain. External canister filters sit outside the tank and are generally used on larger aquariums, offering more filtration capacity and media space than an internal unit of a similar price.

Sponge filters are gentler and are often chosen for tanks with fry, shrimp, or fish that struggle with strong flow, since they filter effectively without creating the current a canister or larger internal filter would. Hang-on-back filters sit on the rim of the tank and are a common middle option for medium-sized setups.

Sizing and flow rate

A filter is generally rated by how many litres of water it can process per hour, and the common guidance is to look for a filter capable of turning over several times the tank's total volume each hour rather than matching it exactly. Manufacturers usually state a suitable tank size range on the packaging as a starting point.

Flow rate matters as much as raw capacity, particularly for fish that prefer calmer water. A powerful filter in a tank of slow-moving or long-finned fish can create more current than they are comfortable swimming against, so matching flow to the fish you keep matters as much as matching capacity to tank size.

Keeping a filter working properly

Rinse filter media in old tank water rather than tap water when cleaning it, since tap water's chlorine can kill the beneficial bacteria the filter is hosting. Stagger replacement of different media types rather than changing everything at once, to avoid disrupting the biological filtration all at the same time.

A filter that has stopped or slowed noticeably is worth checking promptly, since fish depend on continuous filtration rather than an occasional boost — a tank left without working filtration for any length of time can deteriorate faster than most owners expect.

What we stock

Everything below is in stock with us, priced with delivery worked out, and packed and posted by hand in the UK.

Large Aquarium Maintenance Set

A 48-inch condensation tray, a 22-inch glass scraper, a digital thermometer and 250ml of AquaSafe for a big tank.

£32.49

Supa Aquarium Net

Supa aquarium nets in coarse and fine meshes for fish catching and tank maintenance

From£1.65

Choose options

Marina Digital Aquarium Thermometer

A digital aquarium thermometer that mounts on the outside of the glass and reads easily

£3.49

Marina Deluxe Aquarium Scraper

A four-in-one aquarium tool: two pad faces, a scraper blade and a planter fork

£8.79

Marina Floating Aquarium Thermometer

A floating aquarium thermometer with a safe-zone marker for tropical fish

£3.49

Supa Synthetic Filter Wool

25g of synthetic filter wool for aquarium and pond filters, traps fine particles

£3.29

Supa Activated Filter Carbon

Activated carbon filter media for freshwater, marine and pond filtration systems

£5.45

Supa Filter Carbon

150g of activated carbon for aquarium and pond filters, replace every four weeks

£3.35

Marina I25 Internal Filter

A compact internal filter for goldfish or tropical tanks up to 25 litres

£25.29

Fluval Mini Underwater Filter

A 200L/h underwater filter for aquariums up to 45L, rated at 5W

£29.99

Fluval U3 Filter Foam Pad

Fluval U3 foam pads that catch large debris as the filter's first mechanical stage

£7.89

Fluval U1 Filter Foam Pad

A foam filter pad made specifically to fit the Fluval U1 underwater filter

£6.09

Browse all of Aquarium →

Common questions

What size filter do I need for my tank?: Look for a filter rated to turn over several times your tank's total water volume each hour, and check the tank size range stated on the packaging as a starting point. Bigger is not always simply better, since flow rate needs to suit the fish you keep too.
How often should I clean an aquarium filter?: It depends on the filter type and how heavily stocked the tank is, but rinsing media in old tank water periodically rather than replacing it all at once is the general approach, since this avoids disrupting the bacteria colony the filter hosts.
Why does my new filter need time before the tank is ready for fish?: A filter's biological role depends on a colony of bacteria that takes time to establish, a process often called cycling. Adding fish before that colony is established can expose them to waste levels the filter is not yet able to process.
Can a filter be too powerful for my fish?: Yes, particularly for fish that prefer calm water, such as some long-finned or slow-swimming species. A filter's flow rate matters as much as its capacity, so matching it to the fish you keep is worth checking rather than assuming stronger is always better.

Written by Matt Garnett, who runs Giddy Pets and is a lifelong pet owner — not a vet. This is general information, not veterinary advice, and it cannot replace an examination by the vet who knows your animal. How we write these guides.