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.
Tetra Goldfish Flakes
Complete goldfish flakes in several pack sizes, with a shared formula across the family
From£3.02
Tetra Goldfish Granules
Complete granules for goldfish, with a shared formula across both pack options
From£3.98
Tetra Prima Mini Granules
Slow-sinking granules for mid-water and bottom-feeding fish, blending herbivore and omnivore diets
£6.10
Tetra Aquasafe
A water conditioner that neutralises chlorine and heavy metals, with an added vitamin B mix
From£6.41
Tetra Pond Fish Food Variety Sticks
Complete Tetra Pond Variety Sticks in three colours, across four pack sizes
From£6.41
Tetra Koi Pond Fish Food Sticks
Complete fast-soaking Tetra Pond Koi Sticks, available in four pack sizes
From£6.41
Tetramin
Complete TetraMin flake food for ornamental fish, offered across several pack sizes
From£2.39
Tetra Temperate Flakes
Complete flakes for subtropical fish kept at 18–23°C, with vegetable ingredients
£6.99
Tetra Pond Fish Food Sticks
Complete fast-soaking Tetra Pond Sticks for pond fish, in seven pack sizes
From£4.77
Tetra Japan
Sinking dry pellets for fancy goldfish, with pack-specific label information shown clearly
From£4.56
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.
Related
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.