How to Choose the Right External Filter for Your Aquarium

By Ferplast U.K. | Published: 2026-10-13

Category: Buying Guides

Pick an external filter by matching flow rate to tank volume, planning media stages, and checking fit. Use this guide to size a canister filter without overspending.

Quick answer

Choose an external filter rated for roughly four to five times your tank's volume per hour, then confirm the canister fits your cabinet and your plumbing. Prioritise a model with at least three media stages so you can run mechanical, biological, and chemical filtration separately. If you cannot find a published flow rate, treat that as a missing specification and ask the retailer before buying.

Match flow rate to tank volume, not just tank length

Flow rate is the single most useful number when comparing external filters. A common starting rule is to aim for four to five tank turnovers per hour. For a 100-litre tank, that means a filter rated around 400–500 litres per hour. Heavily stocked tanks, goldfish, and messy eaters sit at the higher end; lightly stocked tanks with small fish can sit at the lower end.

Manufacturer flow ratings are usually measured without media and without lift height. Real-world flow drops once you add media and push water up from a canister on the floor. If your filter sits far below the tank, choose the next size up rather than assuming the box figure is what you will see at the outlet.

Plan your filter media setup before you buy

A useful external filter gives you room for three jobs: mechanical filtration to trap particles, biological filtration for beneficial bacteria, and chemical filtration for removing discolouration or medication. Canisters with separate trays let you rinse one stage without disturbing the others, which protects the bacteria colony.

Avoid buying a filter that only holds one type of media. You will end up cleaning it too aggressively and losing biological capacity. Look for a model with at least two or three media compartments, and check that replacement sponges and prefilters are available as spare parts.

Check cabinet space, hose routing, and access

Measure the inside of your cabinet before ordering. External filters need clearance above the unit for the hoses and primer, plus space at the front to slide trays out. A canister that fits with two centimetres to spare will be frustrating to maintain.

Also check hose diameter and intake strainer size against your tank. If you run a lid or a tight hood, confirm the pipework passes through existing cut-outs. For tanks with small pets or children nearby, consider how the intake is guarded.

Decide on electronic control only if it solves a real problem

Some external filters add electronic control, microchip reading, or app connectivity. These features help when you have a specific need, such as keeping a pet out of a room or monitoring flow. They also add cost and another part that can fail.

If you only need reliable mechanical and biological filtration, a simple manual canister is easier to maintain and cheaper to replace. Spend the difference on quality media instead.

Decision rules for sizing and buying

Rule 1: Tank volume first: Multiply your tank's litres by 4 to get a minimum flow target, then round up to the nearest available model. For a 60-litre tank, aim for at least 240 litres per hour. If the manufacturer does not publish a flow rate, do not buy it.
Rule 2: Media trays over extra gadgets: Choose a canister with at least three separate media compartments. This lets you clean mechanical sponges without rinsing the biological media. Fewer than two compartments means more frequent full cleanings.
Rule 3: Measure the cabinet twice: Leave at least 5 cm above the filter for hose connections and 10 cm in front for tray removal. Check that the intake and outlet fit your tank's cut-outs. A filter that barely fits will be skipped at maintenance time.
Rule 4: Check spare part availability: Before buying, confirm you can order replacement sponges, prefilters, and impellers for that model. Genuine spare parts keep flow rates stable over years. No spare parts means a disposable filter.

Product facts that affect your decision

SWING MICROCHIP LARGE: SWING MICROCHIP LARGE is an electronic cat flap for large cats and small dogs with a special antenna that detects and decodes a pet's microchip to unblock the flap. If your aquarium sits in a room shared with pets, this shows how selective-entry hardware works: it only opens for an animal with a registered microchip, so it prevents unwanted animals from entering. It is not an aquarium filter, but it is relevant if you need to control which pets reach the tank area.

SELTZ D Prefilter: The SELTZ D Prefilter is a genuine spare part that captures larger particles before they reach the main filter, helping to maintain airflow and extend the life of your equipment. It is compatible with SELTZ D models in sizes 6000/1600-9000/2400 D AC (SKU XP2108) and 12000/3200 D AC (SKU XP2109). This illustrates the value of a prefilter stage: catching debris early reduces how often you clean the main filter. When choosing an external aquarium filter, look for a model that offers a replaceable prefilter or sponge, because that same principle keeps your biological media cleaner for longer.
SOFT MINI BONES: SOFT MINI BONES are semi-moist dog treats with tripe, designed as a reward during training or a snack between meals. Not a filtration product, but useful context: treats and food rewards can end up in the tank if you feed pets near the aquarium. Keeping treats like these away from the tank reduces the organic load your external filter has to prima-85d94314cf9b0d41fe0cb257-2206575">handle.

Is an external filter right for your setup?

Best for

Tanks from 60 litres upward where you want hidden equipment and flexible media
Keepers who want to run separate mechanical, biological, and chemical stages
Aquariums in living areas where a quiet canister in the cabinet is preferred over a noisy hang-on filter

Not for

Nano tanks under 30 litres where a small internal filter is simpler and cheaper
Tanks with no cabinet or floor space for hoses and a canister
Keepers who want to clean filtration without lifting a lid or opening a cabinet