What's the difference between mechanical and biological filtration?
Written by
Susan Taylor
Reviewed by
Prof. David Walsh, Ph.D.Mechanical and biological filtration have different jobs in your aquarium. Mechanical filtration actually catches things like uneaten food and fish waste (these are visible items). Biological filtration contains beneficial bacteria that digest toxic ammonia and nitrites (these are chemicals that you can't see). Both are crucial in maintaining healthy water; however, they serve different functions.
Mechanical Filtration
- Traps solid particles through physical barriers
- Uses filter floss, sponges, or pads as media
- Prevents debris from clouding water
- Requires frequent cleaning or replacement
Biological Filtration
- Breaks down invisible toxins with bacteria
- Requires porous media like ceramic rings
- Converts ammonia to nitrites then nitrates
- Needs stable colonies preserved carefully
These filtration methods work together in almost all aquarium systems. Mechanical filtration removes solids before they can decompose and contaminate the water. This protects the biological media from clogging. Then, biological filtration can process the dissolved toxins that the mechanical method cannot handle. Their cooperation maintains a balanced quality of water.
Maintain these differently. When restrictions occur, replace the filter floss or a similar mechanical medium weekly. Rinse the sponge pads out in the removed tank water. For biological media (such as ceramic rings), only gently swish in aquarium water during changes. Never use tap water, which kills the essential bacteria.
Avoid widespread all-at-once media replacement. I made this mistake once and experienced a dangerous spike in ammonia levels. Now I stagger maintenance and leave my biological media alone. You must balance mechanical cleaning to support biological efficiency, without compromising your bacterial colonies.
Read the full article: Aquarium Filter Types: Your Essential Guide
