How to Choose an Aquarium Filter
Compare hang-on-back, canister, sponge and internal filters, understand the three jobs of filtration — mechanical, biological and chemical — and size a filter to your tank volume and stocking using turnover rate.
The three jobs of a filter
An aquarium filter does three separate jobs at once, and it is worth knowing which one you are really buying for. Most fish keepers care most about the second — the invisible biological one that keeps the water safe to breathe.
- Mechanical filtration — water passes through sponge or floss that traps particulate waste such as uneaten food, faeces and debris, clearing the water column.
- Biological filtration — beneficial nitrifying bacteria (Nitrosomonas, Nitrospira, Nitrobacter) colonise the media and convert toxic ammonia to nitrite and then to far less toxic nitrate; this is the heart of the nitrogen cycle.
- Chemical filtration — media such as activated carbon adsorb dissolved wastes and toxins; carbon stops working once its pores fill, so it must be replaced periodically.
Types of filter
| Filter type | Best suited for |
|---|---|
| Sponge / corner (air-driven) | Small tanks, fry, quarantine and hospital tanks; gentle flow, cheap, strong biological media |
| Internal power filter | Small to medium tanks; sits inside the tank and is easy to fit |
| Hang-on-back (HOB) power filter | The most common everyday filter; hangs on the tank rim with easy media access |
| Canister (external) | Medium to large or heavily stocked tanks; sealed external unit with large media capacity |
| Undergravel | A porous plate under the gravel with uplift tubes; uses the substrate itself as biological media |
| Wet/dry (trickle) | Water cascades over media in air, boosting oxygen and bacterial activity; common on larger and marine systems |
Filters are rated by the flow they move, in litres or gallons per hour (LPH/GPH), and manufacturers publish a tank-size range for each model. The practical guideline is that the filter should turn over the whole tank volume several times every hour. High-flow reef systems, for example, commonly aim for around 10x turnover — ten times the tank's volume in gallons per hour, which is roughly one complete turnover every six minutes. Bigger, messier or heavily stocked tanks need proportionally more filtration, so when a tank falls between two rated models it is safer to choose the larger one. Adding an external filter also increases total system water volume, which further dilutes waste.
Sources: en.wikipedia.org , en.wikipedia.org (CC BY-SA 4.0), en.wikipedia.org