Filters & flow

Choose, install or troubleshoot your filter

Freshwater aquarium filtration has two connected jobs: moving water through the intended media and supporting the biological processing of waste. A new purchase, a maintenance job and an urgent loss of flow need different starting points. Use the route that matches the problem rather than buying a higher-rated filter before checking the installation.

• Choosing a format: compare internal and external filters after measuring actual water volume and service space. Hang-on-back filters need rim, lid and rear-clearance checks; sponge filters need a suitable continuing air supply.
• Installing or changing equipment: follow the exact water path, arrange canister media correctly and use the model's priming procedure. Fresh rings or foam are not equivalent to established media. Plan a mature-media transfer where appropriate and test afterwards.
• Flow or noise has changed: start with low-flow checks or noise checks. If water is escaping, use the external-filter leak guide and address electrical safety before opening connections.

After any substantial intervention, confirm actual return flow, inspect disturbed joints and check ammonia and nitrite as the situation requires. Clear water alone cannot show whether biological filtration has recovered. Keep the water-testing hub beside the equipment instructions.

Choose a filter you can maintain

An internal filter keeps the equipment in the aquarium and avoids external hoses. It uses swimming space, so check its dimensions and how easily you can remove its media. An external canister frees space inside the tank and can offer more media room, but needs cabinet access, a safe hose route and regular seal checks.

For a canister route, the EF-150 below illustrates the extra hoses and outside space you need to plan for. Its upper litre rating is a maker’s limit, not a promise for every stocking level.

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Keeping the main filter outside the tank

AllPondSolutions EF-150 external filter

An external canister with supplied hose, media and a spray bar. It frees space inside the aquarium, while adding hoses, connections and a canister that need accessible positions.

• 400 L/h rated output
• Up to 150 L maker rating
• Hose, media and spray bar included

Suits

A setup with room outside the aquarium for the filter and a safe, accessible hose route.

Keep in mind

• Check the manual’s installation-height and hose requirements.
• Inspect connections and seals; external filtration adds potential leak points.

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An air-driven sponge filter creates gentle movement and avoids an exposed motor intake. It needs a suitable air pump, airline and, where required, a check valve. Consider the bubbling and pump position if the aquarium is close to where you sleep or work.

Read the rating in context

Work from usable water volume, not the tank's headline capacity alone. Substrate, hardscape and an unfilled top reduce the water actually held. A maker's upper aquarium rating is a starting point; it does not account for every fish or feeding routine.

Check the model's rated flow, media capacity, adjustment options and spare-part availability. Installed flow can differ from the quoted pump figure. Hose runs, media and accumulated debris all affect what reaches the tank. For example, Fluval lists its U2 up to 110 litres; that alone is not a promise that it suits every aquarium of that volume.

For an internal filter, compare media access with the ongoing cost of consumables. The U2 offers adjustable return options; the Bio-CF1 uses a cartridge system for a smaller maker-rated range.

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Adjustable internal filtration

Fluval U2 internal filter

A three-stage internal filter with a flip-top lid and a choice of outlets. It is worth considering when you want accessible media and control over how the return water moves.

• Up to 110 L maker rating
• 400 L/h rated output
• Three-way flow control

Suits

An aquarium where the unit fits without crowding the swimming space and the flow can be set for the fish.

Keep in mind

• The upper litre rating is not a stocking guarantee.
• Takes space inside the tank; check room to lift out the media.

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If changing cartridges appeals, check what the replacement pack costs and which biological media you will keep established. This is the current Bio-CF1 version, including Coir Tech cartridges.

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A cartridge-based small-tank option

Interpet Bio-CF1 internal cartridge filter

The Bio-CF1 uses Coir Tech cartridges alongside ceramic biological media. It offers a different maintenance format from a filter built mainly around reusable sponge blocks.

• 36–68 L maker range
• 320 L/h rated flow
• Coir Tech cartridge system

Suits

A small aquarium where the maker’s 36–68 litre range fits the setup and cartridge replacements suit your routine.

Keep in mind

• Budget for the correct replacement cartridges.
• Keep established biological media; do not replace every filter surface together.

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Keep useful circulation without buffeting fish

Look at the water surface and the places where debris collects. Adjust an outlet or spray bar to spread movement rather than simply directing the strongest jet across the tank. Choose an intake that suits small livestock, and make sure any prefilter sponge can be cleaned without becoming a persistent blockage.

If flow suddenly drops, inspect the intake and the maker's maintenance instructions. Switching to a bigger filter is not the first answer to a blocked sponge or dirty impeller.

A corner layout is another way to distribute the return. The BioPlus 100 lets you remove its media modules without taking out the pump section. Check its height against the actual water depth first.

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A corner-mounted filter

OASE BioPlus 100 internal filter

A corner filter with removable media modules and integrated outlet nozzles. The filter section lifts off for cleaning while the pump section can stay in position.

• Up to 100 L maker rating
• 500 L/h maximum flow
• 120 × 110 × 270 mm

Suits

Layouts with a free corner and enough access above the filter to remove its modules.

Keep in mind

• Not the BioPlus Thermo: a heater is not included with this model.
• Its 270 mm height needs checking against your usable water depth.

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Look after the media that carries the biology

Mechanical media catches debris; biological surfaces support the filter's waste-processing community. Some sponges do both jobs. Clean established biological media gently in removed aquarium water, and avoid replacing all of it together. Keep to the model's water path when fitting different media.

Follow the manufacturer guidance on cleaning filters, and check water results after a substantial change. Our water and testing section explains what to measure alongside your maintenance routine.

Choose a filtration guide

Start with choosing a filter for actual volume and fish if you are planning equipment. It connects usable litres, waste load, media and circulation before you compare products.

Choose a format

• Best internal aquarium filters: compare return options, corner layouts, lift-out sponges and cartridges.
• Best external aquarium filters: compare cabinet access, media space, pump figures and replacement compatibility.
• Internal vs external aquarium filters: decide where the equipment and maintenance work fit.
• Best sponge filters for freshwater aquariums: plan the sponge assembly and separate air supply together.

Understand and maintain the media

• Aquarium filter media explained: understand foam, floss, ceramic media and chemical stages.
• When to replace aquarium filter media: distinguish a cleaning job from a genuinely worn or exhausted component.

Solve a circulation problem

• Why your filter has low flow: check the intake, water path, hoses, priming and prescribed pump maintenance.
• Why your filter is noisy: separate vibration, trapped air, splashing and mechanical noise.
• Reducing flow without stopping circulation: spread a strong return using compatible fittings and permitted controls.

Clean established media carefully

• Cleaning Aquarium Filter Media: Clean the stage that needs attention while retaining established biology.

For air-driven sponge filters, use Air & circulation to plan the pump, valve and backup supply. The check-valve guide covers safe airline routing.

Install accessories and service a canister

Choose the guide for the exact decision or service task. For an active leak, start with safe isolation and water control before investigating replacement parts.

Choose and distribute the return

• Best Hang-On-Back Aquarium Filters for UK Tanks: Compare rim, cover, electrical-version and maintenance requirements.
• Aquarium Filter Intake Guards and Prefilter Sponges: Match the whole intake and keep protection accessible for cleaning.
• Aquarium Spray Bars vs Directional Outlets: Choose the return pattern and exact supported accessory.

Arrange and renew the right media

• How to Arrange Filter Media in a Canister Filter: Follow the actual water path and the model-specific basket layout.
• Activated Carbon in a Freshwater Aquarium Filter: When It Helps: Use carbon for a defined adsorption task while preserving biological filtration.

Fill, inspect and maintain the equipment

• How to Prime an External Aquarium Filter: Fill and vent the supported installation before powering the pump.
• External Aquarium Filter Leaks: Seal, Hose and Connection Checks: Isolate an active leak safely and identify the actual wet connection.
• Cleaning an Aquarium Filter Impeller and Shaft: Remove deposits without damaging model-specific shafts or supports.
• Cleaning External Aquarium Filter Hoses: Match the brush to the hose bore, then restore and check the connections.

Common questions

Where should I start when choosing a filter?

Establish actual water volume, adult livestock and feeding load first. Then compare media capacity, physical access and appropriate return control rather than treating the largest litre rating as a complete selection method.

Does a bigger filter mean cleaner water?

It may provide more media or circulation, but it cannot replace suitable stocking and maintenance. Check maturity, the intended water path and actual aquarium results; excessive local current can be an additional problem.

Can I change all the filter media together?

Avoid discarding all established biological material at once. Identify each stage, follow model instructions, replace only what needs it and monitor ammonia and nitrite after substantial changes or interruption.

Should a filter run continuously?

Filtration should normally run continuously. If noise or current makes that difficult, investigate the installation and correct the underlying issue rather than adopting routine overnight shutdown as a solution.

How do I choose between a spray bar and a directional outlet?

Assess the actual return pattern, fish and layout, then confirm the exact accessory fits the filter. Distributing a jet can improve placement without assuming every spray bar reduces total flow by the same amount.

Does washing media mean it needs replacing?

No. Identify the stage and its condition before deciding. Appropriate gentle cleaning and replacement are different tasks; preserve established biological capacity and follow the exact filter and media instructions.