A tank can look clear while its water quality is moving in the wrong direction. Fish waste, uneaten food and decaying plant matter are not always visible, but they can quickly affect ammonia and nitrite levels. This aquarium filter guide explains what a filter actually does, how to choose the right type and size, and how to look after it without upsetting the biological balance your fish rely on.
What an aquarium filter needs to do
A good filter is not simply there to make the water look tidy. Its main job is to provide a place for beneficial bacteria to live. These bacteria process toxic waste produced by fish and convert it into less harmful compounds as part of the nitrogen cycle.
Most filters handle three forms of filtration. Mechanical filtration catches physical debris such as fish waste, plant fragments and floating particles. Biological filtration provides media with plenty of surface area for beneficial bacteria. Chemical filtration uses materials such as activated carbon or specialist resins to remove certain dissolved substances from the water.
For most established freshwater community aquariums, biological filtration is the priority. Mechanical media keeps the water presentable and prevents debris building up around the tank. Chemical media can be useful for a particular purpose, such as removing medication after treatment, but it is not a substitute for regular maintenance or water changes.
Aquarium filter guide: choosing the right type
The right filter depends on tank volume, livestock, planting, stocking level and the amount of maintenance you are comfortable carrying out. A lightly stocked planted aquarium has different needs from a tank with goldfish, large cichlids or messy-feeding species.
Internal filters
Internal filters sit inside the aquarium and are a practical choice for many small and medium tanks. They are usually straightforward to fit, affordable and easy to access for routine cleaning. Most use a sponge or cartridge arrangement that provides mechanical and biological filtration.
The trade-off is that they take up room in the tank and may offer limited media capacity compared with an external unit. They are well suited to a small tropical community aquarium, quarantine tank or starter set-up, provided their flow rate matches the tank and fish species.
Hang-on-back filters
Hang-on-back filters, often called HOB filters, hook over the rear of the aquarium. They are easy to remove for maintenance and leave more space inside the tank than an internal filter. Their waterfall return can help with surface movement and oxygen exchange.
They are less common on some UK aquarium set-ups than internal and external options, and they need enough clearance behind the tank. Check that the filter can be fitted safely with the aquarium lid in place and that the water level will not fall below the manufacturer’s minimum line.
External canister filters
External filters sit outside the aquarium, normally in the cabinet below. Water travels through intake and outlet pipes, passing through baskets of media before returning to the tank. They offer high media capacity and are often the best option for larger aquariums, heavily stocked tanks and owners who want more control over the media used.
They cost more initially and need careful fitting. Hoses, seals and taps should be checked during maintenance, particularly after the filter has been moved. However, a correctly sized external filter can provide reliable, flexible filtration for a long period.
Sponge filters and air-driven filtration
Sponge filters are powered by an air pump. They are simple, gentle and particularly useful in fry tanks, shrimp tanks, hospital tanks and aquariums holding fish that dislike strong current. The sponge provides a useful biological surface and is safe for tiny fish and invertebrates.
On their own, they may not provide enough mechanical filtration for a larger or heavily stocked aquarium. They are often best used as a supplementary filter or for specialist set-ups where gentle water movement matters more than polished-clear water.
Undergravel filters
Undergravel filters pull water through the substrate, using the gravel bed as biological media. They can work well in a simple, lightly stocked aquarium, but they are harder to clean thoroughly and are not always suitable for planted tanks with nutrient-rich substrate. If debris is allowed to settle beneath the gravel, maintenance becomes difficult. For most modern community tanks, internal or external filtration is easier to manage.
Match filter capacity to the aquarium
Manufacturer guidance will normally state a recommended tank size and flow rate. Treat this as a starting point rather than a guarantee. A filter rated for a 100-litre aquarium may be adequate for a lightly stocked tropical tank, but it could be underpowered for goldfish or larger species that produce more waste.
As a general working guide, aim for water turnover of around four to six times the tank volume per hour in a typical freshwater community aquarium. A 100-litre tank may therefore need a filter delivering roughly 400 to 600 litres per hour once media and normal use are taken into account. Marine tanks, cichlid tanks and goldfish aquariums may need more filtration and circulation.
Do not choose on flow rate alone. Media volume matters because beneficial bacteria need somewhere to colonise. A powerful small filter can create excessive current while still holding little biological media. Conversely, a larger filter with adjustable flow can give useful media capacity without making the aquarium uncomfortable for slower-moving fish.
Watch your fish after fitting a new filter. Fish constantly struggling against the current, hiding behind décor or being pushed around the tank may need reduced flow or a redirected outlet. Bettas, long-finned fish and some calm-water species often prefer gentler movement, while river species generally tolerate more circulation.
Choose media for the job
Filter media is often sold in cartridges, pads, sponges, ceramic rings, biological balls and loose chemical media. The best arrangement usually moves water from coarse mechanical media through finer mechanical media and then across biological media.
Coarse sponge catches larger particles and protects finer materials from clogging too quickly. Fine floss or polishing pads can improve water clarity, although they need replacing or rinsing regularly. Ceramic rings, sintered glass and porous biological media provide ample surface area for bacteria and are especially useful in external filters.
Activated carbon has a place, but it is commonly overused. It can help remove dissolved impurities, odours or treatment residues, yet it becomes exhausted and needs replacing. If there is no clear reason to use it, that space can often be used for additional biological media instead.
Never replace all filter media at once unless there is a genuine contamination issue and you are prepared to manage the aquarium as a new cycle. Throwing away mature sponges and media can remove much of the bacterial colony that keeps toxic waste under control.
Maintain the filter without crashing the cycle
A filter should be maintained when its flow slows, debris is visible, or routine checks show it needs attention. The interval varies. A small internal filter may need attention every few weeks, while a well-sized external filter may run longer between services. Waiting until water flow has almost stopped is not good practice, as trapped waste can reduce performance and strain the pump.
When cleaning sponges or biological media, rinse them gently in a bucket of old aquarium water removed during a water change. Do not wash them under a hot tap or use cleaning products. Chlorine and temperature changes can damage the beneficial bacteria you are trying to protect.
Replace fine pads when they no longer rinse clean, but keep mature biological media wherever possible. Clean the impeller and impeller chamber periodically, as slime and grit can cause rattling, reduced flow or a failure to restart after maintenance. Check intake strainers, spray bars and hoses for blockages as well.
A filter does not remove the need for water changes. Regular partial water changes remove nitrate and dissolved waste that ordinary biological filtration does not fully eliminate. Use a suitable water conditioner when adding tap water, and test the water regularly, especially in new or newly altered aquariums.
Common filter problems and practical checks
Cloudy water soon after a new aquarium is set up is often a bacterial bloom rather than a failed filter. Avoid adding more fish, keep feeding modest and allow the tank to cycle properly. Testing ammonia and nitrite is more useful than repeatedly changing filter media.
If water has turned green, excess light and available nutrients are usually encouraging algae. Reduce unnecessary lighting time, avoid overfeeding and review water-change routine. A filter can collect suspended algae, but it will not solve the cause on its own.
A noisy filter may have trapped air, a dirty impeller or worn parts. Switch it off safely, check the manufacturer’s fitting instructions, clean the impeller area and make sure external-filter hoses are properly primed. If the unit is leaking, isolate it immediately and inspect seals before restarting.
For new fishkeepers, the most reliable route is usually a properly sized internal or external filter with sponge and biological media, combined with sensible stocking and regular water changes. Jalex Pet Products can help make routine aquarium care easier by keeping practical fishkeeping essentials together with the wider pet supplies many households already need.
Choose filtration for the fish you keep, not just the litres printed on the box. A filter that is easy to maintain, holds enough biological media and produces suitable water movement will do more for a stable aquarium than a complicated set-up that never receives proper care.
