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Ask an experienced fishkeeper what the most important piece of kit in the tank is, and they won’t say the heater or the light — they’ll say the filter. A good filter is the difference between a stable, healthy aquarium and a constant battle with cloudy water and dying fish. Yet it’s also the piece of equipment most beginners misunderstand, because its most important job is invisible.
This guide explains what a filter actually does, how to choose the right type and size for your tank, and which filters are genuinely worth buying in the UK in 2026 — from a nano shrimp tank to a big cichlid setup. Get this decision right and most of fishkeeping’s common problems simply never appear.
- The filter is the most important piece of equipment in the tank — it houses the beneficial bacteria that keep your fish alive, not just the sponge that clears the water.
- Match the filter type to tank size: internal or hang-on-back for small-to-medium tanks, canister (external) for tanks above roughly 120 litres, sponge for shrimp, betta and fry tanks.
- Aim for a filter rated to turn over 4–6 times your tank volume per hour, and size upward because manufacturer flow ratings are optimistic.
- Never replace all your filter media at once or rinse it in tap water — you'll destroy the bacterial colony and trigger a mini-cycle.
- The Fluval range covers most needs well; the Oase BioMaster is the pick for keepers who want a built-in heater and easy maintenance.
What a filter really does (it's not what you think)
Most people assume a filter’s job is to clear the water — to catch visible muck so the tank looks clean. That’s part of it, but it’s the least important part. The filter’s essential job is biological: it provides a home for the colonies of beneficial bacteria that convert toxic fish waste into far less harmful compounds. This is the nitrogen cycle, and it’s what actually keeps your fish alive.
Fish produce ammonia, which is highly toxic. Beneficial bacteria living in your filter media convert that ammonia to nitrite (also toxic), and a second group converts nitrite to nitrate (far safer, removed by water changes). Those bacteria live overwhelmingly in the filter, not the water — which is why the filter, and specifically its biological media, is the beating heart of the tank. Understand this one idea and everything else about filters makes sense.
It also explains the golden rule of filter maintenance: never clean your filter media in tap water, and never replace it all at once. Tap-water chlorine kills the bacteria, and replacing everything wipes out the colony — either mistake can crash your tank into a dangerous mini-cycle. Rinse mechanical media gently in old tank water, and leave the biological media largely alone.
The four types of filter, and who each suits
Internal filters
A self-contained unit that sits inside the tank, usually powered by a small pump. Internal filters are cheap, simple and perfectly good for small-to-medium tanks up to around 100 litres. They take up a little in-tank space and need cleaning fairly often, but they’re a sensible default for a first community tank.
Hang-on-back (HOB) filters
These hang on the rim and pull water up through media before cascading it back. Hang-on-back filters keep equipment out of the tank, are easy to access and maintain, and offer good media capacity for their size. They suit small-to-medium tanks and are especially popular for planted tanks where in-tank clutter is unwanted.
Canister (external) filters
A sealed canister that sits in the cabinet below the tank, connected by hoses. Canister filters offer by far the most media capacity and the best filtration, run quietly, and free up all the space inside the tank. They’re the right choice for anything above roughly 120 litres and essential for messy fish like oscars and goldfish. See our dedicated canister filter guide for the top models.
Sponge filters
A block of sponge driven by an air pump. Sponge filters give gentle flow and superb biological filtration, and — crucially — they can’t suck up tiny shrimplets or fry. They’re the go-to for shrimp, betta, and breeding tanks, and make an excellent way to instantly cycle a quarantine tank. See our sponge filter guide.
How to size a filter correctly
The rule of thumb is to choose a filter rated to turn over your tank volume four to six times per hour. For a 200-litre tank, that means a filter rated around 800–1,200 litres per hour (LPH). But there’s an important caveat: manufacturer flow ratings are measured with no media and no head height, so real-world flow is always lower — often much lower once the media clogs a little. Always size upward. A slightly oversized filter is rarely a problem (and flow can be reduced), whereas an undersized one leaves you permanently fighting poor water quality.
Two exceptions. Messy, heavily-stocked or large fish need more filtration than the rule suggests — err generously. And gentle-water fish like bettas need less flow, so either choose a smaller filter or plan to baffle the output.
The best aquarium filters in the UK for 2026
Best overall canister: Fluval 07 series
The Fluval 307/407 is the default recommendation for most mid-to-large tanks, and deservedly so: quiet, reliable, easy to prime and maintain, with generous, well-designed media baskets. If you have a tank of 120 litres or more and want one filter that will serve you for years, this is the safe, excellent choice.
Best for easy maintenance: Oase BioMaster Thermo
The Oase BioMaster Thermo adds two things keepers love: a removable pre-filter module that catches debris (so you can clean it without opening the whole canister and disturbing your bacteria), and a built-in heater that removes an ugly device from inside the tank. It’s a premium pick, but for keepers who value clean maintenance and a clutter-free tank, it’s worth it.
Best hang-on-back: Seachem / AquaClear-style HOB
For a small-to-medium or planted tank, a quality hang-on-back filter offers big media capacity, near-silent running and effortless access. The refillable media basket (rather than throwaway cartridges) lets you build a proper media stack and keep your bacteria intact.
Best for nano and shrimp tanks: sponge filter
In a small, shrimp or breeding tank, a sponge filter on a quiet air pump is unbeatable value — gentle, safe for the tiniest inhabitants, and a huge bacterial surface for its size.
Best budget all-rounder: quality internal filter
For a first community tank up to around 100 litres, a well-made internal filter does everything you need at the lowest cost. Choose one rated comfortably above your tank volume and you’ll rarely have water-quality trouble.
At-a-glance comparison
| Filter type | Best tank size | Media capacity | Flow | Best for |
|---|---|---|---|---|
| Internal | Up to ~100 L | Low–medium | Medium | First community tank |
| Hang-on-back | Up to ~150 L | Medium | Adjustable | Planted / clutter-free |
| Canister | 120 L and up | Very high | Strong | Large / messy fish |
| Sponge | Any (nano+) | Medium | Gentle | Shrimp / betta / fry |
Setting up and maintaining your filter
- Layer media in the right order: mechanical (sponge/floss) first to catch debris, then biological (ceramic/porous) media, then chemical (carbon) last if used. See our filter media guide.
- Position for circulation: aim the outflow to move the whole tank and ripple the surface, which also boosts oxygen — see increasing oxygen.
- Clean sparingly and gently: rinse mechanical media in old tank water only when flow drops, and stagger cleaning so you never disturb all the bacteria at once.
- Run it 24/7: a filter switched off for even a few hours starts to lose its bacteria. Fit a non-return valve on any air-driven filter.
Common filter mistakes to avoid
- Buying too small to save money, then fighting cloudy water forever. Size up.
- Replacing ‘cartridges’ on schedule as the packaging suggests — this throws away your bacteria. Use refillable media instead.
- Rinsing media under the tap and wiping out the colony with chlorine.
- Turning the filter off at night — the bacteria need constant oxygenated flow.
- Ignoring flow direction, leaving dead spots where waste settles. In big tanks, add a powerhead.
Frequently asked questions
What is the best type of aquarium filter?
It depends on tank size. Internal or hang-on-back filters suit small-to-medium tanks, canister (external) filters are best for anything above roughly 120 litres, and sponge filters are ideal for shrimp, betta and breeding tanks. Match the type to your tank rather than assuming one is best for everything.
What size filter do I need for my aquarium?
Choose one rated to turn over your tank volume four to six times per hour, then size upward because manufacturer flow ratings are optimistic. For a 200-litre tank that means around 800–1,200 litres per hour, more for messy or heavily-stocked tanks.
How often should I clean my aquarium filter?
Only when flow noticeably drops, and always gently in old tank water — never under the tap. Stagger the cleaning of different media so you never disturb the whole bacterial colony at once.
Should I replace my filter media?
Avoid the throwaway-cartridge approach, which discards your beneficial bacteria. Rinse and reuse mechanical media until it falls apart, keep biological media in place long-term, and only replace chemical media like carbon as needed.
Which aquarium filter is best in the UK?
The Fluval 07 series is the best all-round canister for mid-to-large tanks, the Oase BioMaster is excellent for easy maintenance and a built-in heater, and a quality hang-on-back or internal filter suits smaller tanks. Sponge filters win for nano, shrimp and betta tanks.
Can a filter be too powerful?
Yes, for some fish. Strong flow exhausts gentle-water species like bettas and gouramis and can stress them or damage fins. If a filter is too powerful, reduce or baffle the outflow rather than switching to an undersized filter.
— Jaqline Smalling, FishForever
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