Aquarium Filtration Basics

The three roles of filtration (mechanical, biological, chemical), the common filter types, and how to choose a filter by turnover rate.

What filtration does

A filter keeps aquarium water clear and safe by moving it through media that perform three distinct jobs: mechanical, biological, and chemical. Most aquarium filters combine these roles in a single unit, and the beneficial bacteria living on the media are central to keeping fish healthy. The filter also keeps the water circulating, which distributes heat and oxygen throughout the tank.

Mechanical filtration

Mechanical filtration removes particulate material from the water column. Water passes through mesh-like or sponge media that trap solids such as uneaten food, fish waste, and debris. These trapped solids are then physically removed when the media is rinsed or cleaned, which is why mechanical media needs servicing more often than the other types. It does not break down waste chemically; it simply holds it until it can be taken out.

Biological filtration

Biological filtration relies on beneficial bacteria that colonise the filter media. These bacteria convert toxic ammonia into nitrite and then into less harmful nitrate, completing the nitrogen cycle. Key groups include Nitrosomonas, Nitrospira, and Nitrobacter. Because this bacterial colony takes time to establish and lives on the media surfaces, the biological media should be disturbed as little as possible during routine cleaning.

Chemical filtration

Chemical filtration uses materials such as activated carbon and zeolites to bind dissolved wastes. Activated carbon adsorbs dissolved toxins on its extended porous surface, removing them from the water. Unlike biological media, chemical media has a finite capacity and becomes exhausted over time, so it is replaced periodically rather than simply rinsed.

Common filter types

• Sponge or corner filters — air-driven, suited to small tanks
• Hang-on-back (power) filters — a common external type with strong flow
• Canister filters — sealed external units with large media capacity
• Internal filters — placed inside the tank
• Undergravel filters — the gravel bed acts as a biological medium

Turnover rate

A common guideline for freshwater aquariums is a filter that turns over the tank volume about ten times per hour. For example, a 120-litre (about 26-gallon) aquarium suits an external canister filter rated around 1,200 litres per hour. Reef tanks require far higher flow, often more than twenty times the volume per hour. How the water is delivered also matters, since good all-over circulation is the goal rather than raw flow alone.