A filter does not remove the need for maintenance
A filter traps particles, provides space for useful microorganisms and circulates water. Depending on media, it can also act on selected dissolved substances. This does not make all waste disappear: trapped material still needs removal during maintenance.
The right filter matches livestock, feeding, layout and your ability to service it. Clear water can contain invisible problems. Filtration does not replace water monitoring or biological establishment before adding animals.
Three functions, many devices
Mechanical
Foam and fibrous materials trap particles. Fineness and fouling influence water passage.
Biological
Colonized surfaces support processes including the nitrogen cycle. Preserving established media matters more than making everything look new.
Chemical
Carbon and resins serve targeted purposes according to their properties. Do not add them automatically without identifying what should be removed.
Air-driven sponge filters
An air pump draws water through foam as bubbles rise. The arrangement is simple, and suitable foam can provide useful surface area in some shrimp or fry setups. The whole assembly still needs sizing and intake assessment.
Foam is visible, and noise can come from the air pump or bubbles. Allow removal without lifting the whole layout. Air delivery and surface agitation also matter in a CO₂-injected project.
Internal filters
An internal filter places pump and media inside the tank. It avoids external hoses and can suit installations without cabinet space below. It also occupies display space.
Check flow adjustment, outlet direction, compartment access and intake protection. A filter hidden behind plants is inconvenient if servicing requires uprooting them. Some offer modules or integrated heating; compare exact features rather than range names.
Hang-on-back filters
A hang-on-back filter rests on the tank edge and returns water after media contact. It leaves cabinet space free and often allows convenient top access. It requires room behind the aquarium and compatibility with frames or lids.
Water level affects waterfall noise and return agitation. Check priming instructions and intended power-outage behaviour. Cartridges and permanent media may serve different functions; read the manual before replacing everything together.
External canister filters
A canister sits outside the tank and connects through hoses. It often offers several media compartments and frees display space, but needs room for removal, valve operation and hose cleaning.
Installation height, bends, media and fouling affect operating flow. Check seals and fittings after maintenance. In planted tanks, an inline CO₂ diffuser may fit the return when hose sizes, pressure and equipment instructions are compatible.
Internal, rear and under-tank sumps
A sump or filter chamber directs water through compartments. It may be inside the tank, behind an all-in-one display or below the aquarium with a return pump. Available media space and access can be advantages.
An external sump needs coordinated overflow, return and spare capacity when the pump stops. It is not simply another container under a tank. Follow system documentation and verify intended outage behaviour without improvised plumbing changes.
Advertised flow and actual circulation
Read advertised litres per hour alongside measurement conditions. It does not describe performance after hoses, accessories and media are fitted. Tank volume alone is insufficient too: dense layouts and stocking change requirements.
Observe leaf movement, deposit areas and livestock behaviour. An intake guard for small shrimp can clog quickly; include it in maintenance. Seek effective circulation with suitable refuges, not the strongest possible jet.
Maintain without starting over
Clean mechanical sections according to fouling and instructions. Preserve established biological media; avoid replacing all its supports at once unnecessarily. Keep it wet during normal servicing and follow manufacturer rinsing guidance.
Check seals, valves and water level before restarting. Do not run a pump dry. Afterwards, inspect for leaks and restored flow. Record dates and replacements to distinguish expected changes from new problems.
Choose for the tank, not a universal ranking
In a small shrimp aquarium, gentle protected intake may matter more than completely hidden equipment. In a densely planted tank, media access and distribution around the hardscape become important. In a quiet room, consider pump, air and waterfall noise according to filter type.
Measure available space before buying: behind the tank for a hang-on-back, inside the cabinet for a canister, or within the tank for an internal filter or compartment. Allow room to remove a basket, open a lid or close a valve. A filter with impressive specifications can still be a poor choice if servicing is so awkward that it is constantly postponed.
Compare practical constraints at a glance
| Type | Practical strength | Plan for |
|---|---|---|
| Sponge | Simple, accessible media | Air pump, noise and in-tank space |
| Internal | Compact in-tank installation | Housing access and occupied space |
| Hang-on-back | Media accessible at the tank edge | Rear clearance, water level and lid |
| Canister | Media volume outside the layout | Hoses, valves, cabinet and seal checks |
| Compartment or sump | System-integrated chambers | Usable volume, circulation and technical access |
These pointers do not replace manufacturer dimensions, flow conditions and recommendations. Two filters in the same category may have very different capacities and requirements.
Frequently asked questions
Is a bigger filter always better?
No. Flow, capacity and servicing must fit livestock and layout.
Is carbon mandatory?
No. It serves selected purposes. Choose media for an identified need.
Does CO₂ require a canister?
No. In-tank diffusion can work without one. Inline diffusion needs compatible plumbing.
Should the filter stop at night?
A biological filter normally runs continuously. Do not schedule it like lighting; follow system instructions.
Continue reading
- Aquarium CO₂: regulators, diffusers and adjustment
- Aquarium types: rimmed, rimless or braced?
- Planning a planted aquarium: from idea to planting