Aquarium Filtration Explained: Types of Filters and How They Work

What Are the Three Types of Aquarium Filtration?

Every aquarium filter provides some combination of mechanical, biological, and chemical filtration. Understanding these three processes helps you choose the right filter and maintain it properly. They work together in sequence: water passes through mechanical media first (removing particles), then through biological media (where bacteria process ammonia), and finally through chemical media (which removes dissolved contaminants). For related guidance, see our Fish Tank Setup Guide Uk guide.

Mechanical filtration physically traps suspended particles like fish waste, uneaten food, plant debris, and dust. This is what makes the water look clear. Filter sponges, floss, and pads perform this role. Without mechanical filtration, the water appears cloudy and particles accumulate. However, clear water is not necessarily safe water; mechanical filtration alone does nothing to address the invisible toxins (ammonia, nitrite) that actually harm fish. For related guidance, see our Freshwater Vs Tropical Fish Uk guide.

Biological filtration is the process where beneficial bacteria living on filter media convert ammonia (produced by fish waste and decomposing organic matter) first to nitrite, then to nitrate. This is the nitrogen cycle, and it is the single most critical function of your filter. Without biological filtration, ammonia accumulates to lethal levels within days. Chemical filtration uses activated carbon, zeolite, or specialist resins to remove dissolved organics, medications, discolouration, and odours from the water. It is the least essential of the three but useful in specific situations. For related guidance, see our Aquarium Maintenance Schedule Uk guide.

What Is the Internal Filters: The Budget-Friendly Choice?

Internal filters sit inside the aquarium, typically in a corner, and are the most common type included with UK starter kits. They are powered by a small impeller pump that draws water through filter media (usually a sponge or combination of sponge and carbon) and returns it to the tank. Brands popular in the UK include Fluval U-series, Interpet Internal, and Eheim Pickup.

Advantages of internal filters include low cost (10-30 pounds), easy installation, no risk of leaks outside the tank, and suitability for smaller tanks under 100 litres. They are quiet, require minimal setup, and are perfect for beginners. The main disadvantage is limited media capacity: the small sponge or cartridge holds fewer beneficial bacteria than larger external filters, limiting the biological filtration capacity.

For tanks of 60-100 litres with moderate stocking levels, a good internal filter is perfectly adequate. Choose one rated for your tank volume or larger. Some advanced internal filters (like the Fluval U-series) allow you to customise media, which is a significant advantage over basic models with single-piece cartridges. Avoid filters that require you to replace the entire cartridge, as this discards all your beneficial bacteria. Look for models with reusable sponge media that can be rinsed and reused indefinitely.

What Is the External Canister Filters: Superior Performance?

External canister filters sit outside the aquarium (typically in the cabinet below) and connect to the tank via inlet and outlet hoses. Water is drawn out of the tank, passes through multiple layers of filter media inside the canister, and is returned via a spray bar or outlet nozzle. Popular UK brands include Fluval, Eheim, and Oase.

The primary advantage of external filters is media capacity. A canister holds significantly more filter media than any internal filter, providing superior biological filtration and allowing greater stocking levels. A Fluval 207 (for tanks up to 220 litres) holds over 2 litres of media compared to a few hundred millilitres in a typical internal filter. This additional capacity makes water quality more stable and the system more resilient to fluctuations.

External filters cost more (50-200 pounds depending on size and brand) and require more careful setup to avoid leaks. However, they are quieter than most internal filters, do not take up space inside the tank, and need less frequent cleaning (every 2-3 months rather than monthly). For tanks over 100 litres, or tanks with heavy stocking, an external canister filter is the recommended choice. The initial investment pays for itself in reduced maintenance time and better water quality.

How Should You Choose the Right Filter Size?

The most common filtration mistake is choosing a filter that is too small. Manufacturers rate filters by tank volume, but these ratings assume light stocking. For a realistic setup with a reasonable number of fish, choose a filter rated for at least 1.5 times your actual tank volume. For a 100-litre tank, choose a filter rated for 150 litres or more. For tanks with messy fish (goldfish, large cichlids), aim for 2-3 times the tank volume.

Flow rate is another consideration. The filter should turn over the entire tank volume 4-6 times per hour for tropical community fish, and 6-10 times per hour for goldfish and other heavy waste producers. A 100-litre tank needs a filter with a flow rate of at least 400-600 litres per hour. Note that flow rates decrease as filter media accumulates debris between cleanings, so starting with a higher-rated filter ensures adequate flow even when the media is due for maintenance.

For larger tanks, running two smaller filters rather than one large filter provides redundancy: if one filter fails, the other maintains biological filtration while you address the problem. This is particularly valuable if you go on holiday or are away from home regularly. Many experienced UK fishkeepers run an internal filter as backup alongside their main external filter for exactly this reason.

What Are the Filter Media Types and Maintenance?

Filter media falls into three categories matching the three filtration types. Mechanical media includes sponges (coarse to fine), filter floss, and filter pads. Arrange from coarse to fine in the direction of water flow so large particles are trapped first, extending the life of finer media. Biological media includes ceramic rings, sintered glass (such as Seachem Matrix or Siporax), bio-balls, and purpose-made plastic media with high surface areas for bacteria colonisation.

Chemical media includes activated carbon (removes dissolved organics, discolouration, medication residues), zeolite (absorbs ammonia – useful in emergencies), and specialist resins (for phosphate removal, nitrate removal, or water softening). Carbon becomes exhausted after 4-6 weeks and should be replaced or removed. Many experienced fishkeepers do not use chemical media in their everyday setup, relying instead on mechanical and biological filtration supplemented by regular water changes.

The golden rule of filter maintenance: never replace all media at once. Swap or clean only one type of media per maintenance session, allowing bacteria to recolonise from the remaining media. If your filter uses a single cartridge that must be replaced entirely, consider upgrading to a filter with customisable media compartments. Rinse all media in old tank water removed during water changes, never under the tap. UK tap water chlorine destroys beneficial bacteria on contact.