Internal vs HOB vs canister filter — which one?

Internal: best for tanks under 60 L or if you don't want anything outside the tank. HOB: best all-rounder for 60-200 L tanks — easy access, good biological capacity. Canister: best for 200+ L, planted tanks, or anyone who wants no visible equipment. Sponge filters: best for shrimp tanks, breeding setups, and hospital tanks where gentle flow matters.

What does mechanical/biological/chemical filtration actually mean?

Mechanical = physical sieve catching debris (sponges, floss). Biological = living bacteria converting ammonia → nitrite → nitrate (ceramic rings, bio-balls, also the surface of every other piece of media). Chemical = activated carbon or resin removing dissolved organics (medications, tannins, odours). Biological is the only one that's literally life-or-death for fish.

How often should I clean my aquarium filter?

Mechanical media (sponge, floss): every 2-4 weeks, or when flow noticeably drops. Biological media (ceramic, bio-balls): rarely — only if flow is severely restricted, and only in old tank water. Chemical media (carbon): replace every 3-4 weeks if you use it. The bigger sin is over-cleaning — every wash sets back the bacteria colony.

Can I rinse my filter sponge in tap water?

No. UK tap water contains chlorine OR chloramine which kills filter bacteria instantly. Always rinse media in old tank water you've syphoned out during a water change. A bucket of tank water and a few good squeezes is all the cleaning a sponge needs.

How big a filter do I need for my tank?

Filter flow rate should be at least 4-6× your tank volume per hour. So a 100 L tank wants a filter rated 400-600 L/h MINIMUM. Manufacturers print best-case flow rates measured with empty media — real-world flow drops 30-40% with media installed. Buy bigger than the spec sheet suggests.

Should I run chemical filtration (activated carbon)?

Not routinely. Activated carbon is for specific jobs: removing medication after a treatment, clearing tannin discolouration from new driftwood, or polishing water for a display tank before a show. In a normal maintained tank with weekly water changes, carbon does nothing useful. Don't use it during medication — it removes the medication too.

Why is my filter making bubbling/grinding noises?

Three common causes: (1) air trapped in the impeller chamber — turn filter off and on to release, (2) impeller worn or clogged with debris — clean with cotton bud, (3) intake strainer partially blocked — restricts flow and causes cavitation. If the noise persists after cleaning, the impeller magnet has likely worn and needs replacing.

Can I run two filters on one tank?

Yes, and many experienced fishkeepers do. Two smaller filters give you redundancy (one fails, the other keeps fish alive), let you stagger maintenance (clean one, leave the other untouched for 4 weeks), and double biological capacity. Common pairing: an internal filter + a HOB, or a small canister + an HOB.

What's the difference between flow rate and turnover?

Flow rate is the litres-per-hour the filter pump moves. Turnover is flow rate divided by tank volume — how many times all the water passes through the filter per hour. Aim for 4-6× turnover for community tanks, 10× for cichlid/messy tanks, 2-3× for shrimp/betta tanks (they don't like strong currents).

How long can my filter be off without killing bacteria?

30-60 minutes maximum. Filter bacteria need oxygenated water flow — they suffocate in stagnant water. If your filter has been off for hours (power cut, broken impeller), rinse the media in fresh dechlorinated water before restarting; dying bacteria release ammonia and a stagnant restart can spike a tank.

Do I need a filter for a heavily planted tank?

Yes, even Walstad-method natural tanks benefit from gentle circulation. Live plants consume ammonia directly but don't replace biological filtration — they slow it. A sponge filter or small HOB provides surface agitation (gas exchange) and circulation without the heavy turnover that would disrupt plants.

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Crystal-clear planted aquarium powered by quality biological filtration

Understanding Aquarium Filtration (UK Beginner's Guide)

Kevin

Updated 18 May 20269 min read

3 sources: 1 peer-reviewed, 1 hobbyist ref., 1 regulatory

Why filtration matters more than you think

Most beginners think a filter is a debris catcher. It isn't — it's a life-support system. Inside your filter, billions of nitrifying bacteria^[1] convert toxic fish waste into compounds your fish can tolerate. Without them, ammonia accumulates and kills fish within days. Get filtration right and your tank thrives; get it wrong and no amount of water changes will save you.

This guide is the explanation I wish someone had given me before I bought my first under-spec filter. Everything here is cross-referenced against peer-reviewed microbiology and the most respected hobbyist database^[2].

The three jobs every filter does

Job	What it does	How important	Where it happens
Biological	Bacteria convert ammonia → nitrite → nitrate	Critical — fish die without it	Ceramic rings, bio-balls, sponge surface
Mechanical	Physical sieve traps debris and waste	Helpful — keeps water clear	Coarse → fine sponges, floss
Chemical	Resin/carbon binds dissolved organics	Optional — most tanks don't need it	Activated carbon, Purigen, zeolite

The trap: every beginner guide lists these three as equal. They're not. Biological filtration is what keeps fish alive. The other two are useful but secondary. Protect your biological media first and everything else falls into place.

Mechanical filtration — the sieve

Water flows through layered media from coarse (catches the big stuff) to fine (polishes to clarity). Maintenance is simple but the rules are firm:

Rinse in old tank water only. Squeeze the sponge in a bucket of tank water during your weekly water change. The water can be murky — that's fine
Never rinse under the tap. UK tap water is chlorinated^[3] — instant kill for the bacteria living on the sponge surface
Replace floss when it falls apart. Sponges are essentially permanent — keep them for years
Don't aim for clean. A slightly-clogged sponge filters better than a pristine one — biofilm IS the biology

The single most damaging filter mistake

Rinsing your filter sponge under the tap. The chlorine kills the bacteria you spent 4-6 weeks growing during your cycle. Within 48 hours your ammonia spikes, fish stress, and you're back at square one. Always squeeze in old tank water. This rule is non-negotiable.

Biological filtration — the heart

This is where the nitrogen cycle lives. Two bacterial species do all the work:

Nitrosomonas converts toxic ammonia (NH₃) → still-toxic nitrite (NO₂⁻)
Nitrospira converts nitrite (NO₂⁻) → much-less-toxic nitrate (NO₃⁻) — the Hovanec & DeLong 1996 paper^[1] proved this against the long-held assumption it was Nitrobacter

You remove the resulting nitrate with weekly water changes. That's the whole cycle. Full procedure in the dedicated guide:

→ Fish Tank Nitrogen Cycle UK guide

Where the bacteria actually live

Despite what most beginner guides say, nitrifying bacteria do NOT primarily live in your gravel or on the tank glass. They live on the surface area of your filter media — ceramic rings, bio-balls, and the sponges themselves. The filter is the engine. This is why mature filter media from a friend's tank can cycle your new tank in 1-2 weeks instead of 4-6.

Three golden rules:

Never replace all biological media at once. Swap half, wait a month, swap the rest
Never rinse bio media in tap water. Same chlorine problem as mechanical
Never turn the filter off "just for a few hours." Bacteria start suffocating within 30 minutes without oxygenated flow

Chemical filtration — usually optional

Use carbon for	Don't use carbon for
Removing medication residue AFTER a treatment	DURING medication (it removes the medication)
Clearing tannin colour from new driftwood	Tanks with soft-water species (they like tannins)
Polishing water before a show or photo	Routine maintenance — water changes do this free
Removing odours from established tanks	Long-term — carbon saturates in 3-4 weeks

Saturated carbon stops working and can leach contaminants back. If you use it, replace on schedule. Most well-maintained tanks don't need carbon at all — weekly water changes do the same job.

Filter types — which one for which tank

Type	Best for	Pros	Cons	UK price range
Sponge filter	Shrimp tanks, breeding, hospital tanks, nano	Cheapest, gentle flow, huge bio surface, won't suck up fry	Visible in tank, needs air pump	£8-£20 + pump
Internal filter	Tanks under 100 L, beginners	Simple, all-in-one, easy maintenance	Bulky inside the tank	£25-£60
Hang-on-back (HOB)	60-200 L community tanks	Best all-rounder, easy access, good flow	Visible at back, slight noise	£40-£100
Canister filter	200 L+, planted tanks, premium setups	Maximum capacity, hidden, customisable media	Expensive, harder to clean	£100-£300

For most UK beginners: an HOB on a 100 L tank. AquaClear 50, Fluval Aquaclear 70, or Seachem Tidal 75 are all proven choices.

Buy 1.5× your tank volume — minimum

A filter rated for 150 L on a 100 L tank gives you headroom for inevitable bioload growth (more fish, bigger fish, occasional overfeeding). Manufacturers measure flow with empty media — real-world flow with a full sponge stack drops 30-40%. The "rated for your tank size" filter is actually under-spec'd.

Maintenance schedule

Frequency	Task
Weekly	Check flow rate by eye — should look strong and steady
Every 2-4 weeks	Rinse mechanical sponge in old tank water during water change
Every 3-4 weeks	Replace activated carbon if you're using it
Every 3-6 months	Gentle rinse of bio media in old tank water (only if flow severely restricted)
Every 3-6 months	Clean impeller and intake strainer
Every 6-12 months	Check impeller magnet for wear; replace if grinding starts

Never clean the filter the same day as a big water change

Water changes remove some bacteria from the water column; filter cleaning disturbs the colony on the media. Doing both on the same day removes too much at once and can briefly destabilise your cycle. Spread maintenance across the week — water change Sunday, filter rinse Wednesday.

Common filter mistakes

Mistake 1: Replacing cartridges monthly because the box says so

Pre-made filter cartridges are sold as monthly disposables because that sells more cartridges. The bacteria living on them are perfectly happy to stay for years. Bin the cartridge marketing — keep the sponge, replace only the floss when it disintegrates.

Mistake 2: "My water looks cloudy, the filter must be broken"

Cloudy water is almost never a filter problem. Common real causes: bacterial bloom in a new tank (harmless, clears in 7-14 days), overfeeding (fix the feeding), stirred-up substrate (settles), or a mini-cycle from too-aggressive cleaning (rare but possible). Test water before blaming the filter.

Mistake 3: Turning the filter off at night to be "quiet"

Bacteria suffocate within 30 minutes without flow. A noisy filter wants attention (clean the impeller, check the airline, reposition slightly) — not switching off. A modern HOB or canister should be near-silent; if yours is loud, fix the cause, don't silence the patient.

Mistake 4: Adding a second-hand filter without quarantine

Filter media from another tank IS the fastest way to cycle a new tank — but only if that source tank is disease-free. A second-hand HOB from Facebook Marketplace can bring in ich, internal parasites, or fungal spores. Run media for 4 weeks in a fishless quarantine tank before trusting it.

Mistake 5: Buying the cheapest no-brand filter

The pump fails, the impeller cracks, the flow rate is half the spec. A £25 no-brand "200 L/h" filter is often a real-world £150-L/h that lasts 18 months. The Fluval / AquaClear / Eheim / Oase price premium buys real-world reliability and 10+ year spare-part availability.

Summary

Pick an HOB or canister rated 1.5× your tank size. Protect the biological media at all costs — never tap-water rinse, never replace all at once, never turn off. Rinse mechanical sponge in old tank water every few weeks. Skip chemical filtration unless you have a specific reason. Get those four things right and your filter will outlast your tank.

Fish Tank Nitrogen Cycle UK Guide — what the bacteria in your filter actually do
Complete Tropical Fish Tank Setup — the whole beginner setup including filter choice
How to Acclimate New Fish — drip method for your first stocking day
Tropical Fish Water pH — water chemistry your filter has to keep stable
Browse all tropical fish

Frequently asked questions

For tanks 60-120 L, an internal filter or hang-on-back (HOB) rated 1.5× your tank volume. The Fluval U2/U3 and AquaClear 50/70 are the safest UK choices — both have widely-available spare parts and decent biological capacity. Avoid the cheapest no-brand filters; their flow rate is overstated and impellers fail within months.

Browse our UK live-fish range

From this article straight into the catalogue.

Beginner Fish

The best tropical fish for beginners UK — hand-picked easy-care species that thrive with minimal fuss. Perfect starter community tank candidates.

Community Tank Fish

Community tank fish for sale UK — peaceful, compatible species that coexist happily. Perfect for mixed-species tropical aquariums.

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Premium Tetras for sale UK — Cardinal, Ember, Black Neon, Rummy Nose, Serpae, Diamond, Glowlight and more. Peaceful community schoolers.

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Fancy Guppies (Poecilia reticulata) for sale UK — Moscow, Cobra, Tuxedo, Dumbo Ear and more. Hand-selected, live arrival guarantee.

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Neocaridina Cherry Shrimp for sale UK — Red, Blue Velvet, Yellow, Orange, Green Jade. The best beginner freshwater shrimp, live arrival guarantee.

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Corydoras catfish for sale UK — Bronze, Panda, Pygmy, Sterbai and rare species. Peaceful bottom-dwelling schoolers, live arrival guarantee.

Freshwater Snails

Freshwater aquarium snails for sale UK — Nerite, Mystery, Assassin, Rabbit, Ramshorn. Algae control and interest for any tank.

Sources & further reading

Every claim in this article is backed by a source below. We group them by type so you can judge the weight of each one at a glance.

Peer-reviewed study (1)

[1]
Hovanec, T. A. and DeLong, E. F. (1996). Comparative analysis of nitrifying bacteria associated with freshwater and marine aquaria. Applied and Environmental Microbiology, 62(8). View source
Confirms Nitrospira (not Nitrobacter) is the dominant nitrite-oxidising bacteria in aquarium filters. Drives which bacterial colonies actually develop on filter media.

Hobbyist reference (1)

[2]
(2023). Filtration — choosing and using aquarium filters. Seriously Fish. View source
Cross-checked on flow-rate recommendations, filter-type pros/cons, and maintenance intervals.

Government / regulatory (1)

[3]
(2024). Drinking water standards — UK disinfectants. Drinking Water Inspectorate (DEFRA). View source
Documents that UK tap water contains chlorine or chloramine — both kill filter bacteria on contact. Source for the 'never rinse media under the tap' rule.

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