Types of aquarium filters

Every aquarium filter does the same basic job: move water through media to remove waste. But the four main designs - hang-on-back, canister, sponge, and internal - do that job in very different ways, with real trade-offs that affect your fish, your maintenance routine, and your wallet. This guide covers how each type works, where it excels, and where it falls short.

If you're not yet clear on why filtration matters at the chemistry level, read the nitrogen cycle explained first - it makes sense of why biological filtration is the one you can't skip.

Hang-on-back (HOB) filters

HOB filters clip onto the top edge of the tank. A tube drops into the water, draws it up into a media compartment housed in a box on the outside of the glass, and returns it via a spillway that pours back over the surface. That return spillway is actually one of the HOB's underrated strengths: it produces consistent surface agitation, which drives gas exchange and keeps dissolved oxygen levels high without needing a separate airstone.

Most HOBs use a cartridge system out of the box - a pad with some activated carbon sealed inside. Replace it on a schedule and you throw away your established bacterial colony every time. Swap cartridges for separate mechanical foam and biological media (ceramic rings, bio-balls, or matrix) and a HOB becomes a much more capable filter.

Pros

• Inexpensive - good units start around £20-£40 / $25-$50.
• Easy to service. The media box lifts out with no priming, no disconnecting hoses.
• Good surface agitation included by default.
• Widely available; replacement media is sold everywhere.

Cons

• Visible on the back of the tank - not ideal for display setups.
• Limited media volume compared to canisters of the same price.
• Can rattle or gurgle if the water level drops and the spillway runs dry. Keeping the tank topped up solves this.
• The proprietary cartridge trap: many brands push replacement cartridges hard. Ignore the packaging - use loose media instead.

Best for: Beginners, community tanks up to around 200 L / 55 gal, any setup where easy maintenance is a priority.

Canister filters

Canister filters sit outside the tank - usually in the cabinet underneath - and connect via intake and return hoses. Water is pulled down into a sealed pressurised canister, forced through stacked media trays, and pumped back up into the tank. Because the canister is sealed and submerged in the cabinet rather than exposed to air, they run significantly quieter than HOBs at equivalent flow rates.

The big advantage is media volume. A mid-range canister will hold three or four separate media types - mechanical foam at the bottom, biological ceramic or sintered glass in the middle, and whatever chemical media you choose on top. That flexibility is why experienced fishkeepers tend to graduate toward canisters once their tanks get larger or more complex.

Pros

• High media capacity - more biological surface area means a more stable cycle.
• Hidden from view; only the spray bar or return outlet is visible in the tank.
• Powerful, adjustable flow - a spray bar or lily pipe return can be directed precisely.
• Longer service intervals. Most canisters only need opening every 2-3 months under normal stocking.

Cons

• More expensive upfront - quality units start around £70-£120 / $90-$150.
• Require priming when restarted. Most modern units are self-priming, but it's still a step.
• Messy to service. Disconnecting hoses while avoiding spills takes practice.
• If the seal or tubing fails, a slow leak can go unnoticed inside the cabinet for days.

Best for: Tanks 150 L / 40 gal and up, planted tanks, cichlid setups, any tank where aesthetics matter and you want equipment hidden.

Sponge filters

A sponge filter is a block of open-cell foam connected to a lift tube. An air pump pushes air bubbles up through the tube, which draws water through the sponge by suction. Bacteria colonise the sponge's enormous surface area and handle the biological filtration. The sponge also catches particulate waste mechanically, though it won't polish the water as finely as a canister's multi-stage media stack.

The flow is gentle - genuinely gentle. For fish or invertebrates that live in slow-moving water in the wild, this is a feature, not a limitation. Bettas, dwarf cichlids, most shrimp species, newly hatched fry - all of these can be harmed or stressed by the strong intake current of an HOB or canister. A sponge filter is the correct tool for that job.

Pros

• Extremely cheap - often under £5 / $7 per unit.
• No impeller means nothing to suck up fry or shrimp.
• Gentle, diffuse flow that suits sensitive livestock.
• Excellent biological filtration; the sponge develops a mature colony quickly.
• Easy to seed a new tank - just transfer a seeded sponge from an established tank.

Cons

• Requires a separate air pump and airline tubing - the starter cost isn't quite as low as the unit price suggests.
• Sits inside the tank and is visible.
• Weaker mechanical filtration - fine particles pass straight through.
• In tanks with higher bioloads, you may need two or three sponges running in parallel.

Best for: Breeding tanks, shrimp tanks, hospital and quarantine tanks, betta tanks, nano setups, any tank where fry safety or gentle flow is the priority.

Internal filters

Internal filters sit fully submerged inside the tank, usually suctioned to a back or side panel. A small pump pulls water through foam media and returns it into the tank. They're cheap, simple, and take up minimal space outside the aquarium - which is appealing in small apartments or on tight budgets.

The trade-off is that they occupy space inside the tank, the media volume is small, and the flow rates are modest. For a nano tank under 60 L / 15 gal with a light bioload, an internal filter does the job adequately. In anything larger, you'll start feeling the limitations.

Pros

• Very low cost - often included with starter tank kits.
• Simple setup; no external hoses or cabinet space needed.
• Adjustable flow direction on most models.

Cons

• Takes up space inside the tank that could go to livestock or decor.
• Small media volume limits biological capacity.
• Can be harder to clean - you have to reach into the tank every time.
• Visible and often aesthetically awkward in aquascaped tanks.

Best for: Nano tanks under 60 L / 15 gal, low-bioload setups, quarantine tanks where simplicity matters more than capacity.

Choosing the right filter for your tank

There's no universal best filter type - the right answer depends on tank size, livestock, and how much time you want to spend on maintenance. Here's a practical framework:

• Under 60 L / 15 gal: Sponge filter or internal. Add a second sponge if you're keeping shrimp or breeding.
• 60-150 L / 15-40 gal community tank: HOB, sized up from the rated volume. A sponge filter as a supplementary biological filter is never a bad idea.
• 150 L / 40 gal and above: Canister. Run an additional sponge filter if you're heavily stocked.
• Breeding, hospital, or shrimp tanks: Sponge filter regardless of tank size.

Before finalising your stocking plan, use the free stocking calculator to check whether your filter capacity is realistic for the fish you want to keep. Overstocking is the number one cause of filter failure in the hobby.

What most guides miss: running two filters

Most fishkeeping content treats filter choice as picking one winner. Experienced aquarists often run two filters - not because one is inadequate, but because it solves real problems. Two smaller HOBs give you redundancy: if one needs servicing or fails, the other keeps the biological cycle intact. A canister paired with a sponge filter gives you the canister's mechanical power and the sponge's indestructible biological colony as a backup. When you're heavily stocked or running a species-only tank that demands stable water, redundancy isn't overkill.

This also makes fishroom tasks easier. When you clean one filter, you leave the other running untouched so you never crash the cycle. Keep an eye on your aquarium water parameters after any filter maintenance - ammonia spikes after a thorough clean are more common than most people admit.

Filter media: the part that actually does the work

The filter housing is just a box. The media inside is what performs mechanical, biological, and chemical filtration. Getting the media right matters as much as choosing the filter type.

• Mechanical media (filter foam, filter floss, polishing pads): traps suspended particles. Coarse foam goes first in the flow path to protect finer media downstream. Rinse in old tank water every 2-4 weeks.
• Biological media (ceramic rings, sintered glass like Seachem Matrix, bio-balls): provides surface area for nitrifying bacteria. Never replace all of it at once. Never rinse it in tap water. It's the most valuable thing in your filtration system.
• Chemical media (activated carbon, Purigen, zeolite): optional and situational. Activated carbon removes medications and odours but depletes quickly and needs replacing every 4-6 weeks if used long-term. Most established tanks don't need it permanently.

If you're still cycling a new tank, protect your biological media above everything else. It takes weeks to establish and minutes to destroy.