Why is biological filtration critical?
Written by
Hoang Long
Reviewed by
Prof. David Walsh, Ph.D.Biological filtration provides the foundation of healthy aquarium aquascapes and is where beneficial bacteria live to convert fish waste (ammonia) to less toxic substances. This organic process is known as the nitrogen cycle and is essential to the health of the fish in your aquarium: If they weren't being protected by biological filtration, they would be poisoned by ammonia within hours!
Ammonia Conversion
- Fish waste and decaying matter release toxic ammonia
- Nitrosomonas bacteria convert ammonia to nitrites
- Process begins within 24 hours of adding fish
Nitrite Breakdown
- Nitrobacter bacteria process nitrites into nitrates
- Occurs after ammonia conversion establishes
- Nitrites remain toxic until converted
Nitrate Management
- Plants absorb some nitrates as nutrients
- Water changes dilute remaining nitrates
- Final stage makes water safe for inhabitants
Beneficial bacteria require the right conditions to thrive. They tend to inhabit porous surfaces, such as ceramic rings and bio-balls. These bacteria, while inhibited under low-oxygen conditions, require constant water flow to bring oxygen and ammonia. Temperatures between 65°F and 85°F maintain optimal metabolic activity. The colony fully establishes over 4-6 weeks in new tanks.
Protect bacterial colonies. Never clean biological media with chlorinated tap water. Rinse gently with some of the tank water you just removed if the downflow seems to be lessening. Don't use antibiotic medicines that kill nitrifying bacteria in your tank. Always try to maintain the established biological medium when upgrading a tank filter. I have saved a few tanks by simply placing one of the glands from the so-called biological medium in my new filter.
Test to ensure that your biological filtration is functioning properly. Monitor ammonia and nitrite levels weekly with a liquid test kit. Acceptable levels are 0 ppm ammonia and 0 ppm nitrites. Nitrates should be kept under 40 ppm by regular partial water changes. Any spike is an indicator that your biological filtration has been compromised - remedial action is urgently indicated.
Supplement biological filtration: In extreme situations, use additional biomedia when loading up a tank with a lot of fish. Add bacterial starter cultures to a new aquarium to speed up the cycling. Use pre‐filters to add more colonizable surface area.
Detect signs of biological filtration failure. Ammonia poisoning is characterized by fish gasping at the water surface. Cloudiness in the water column may indicate a bacterial bloom resulting from a disrupted cycle. Lethargy can occur as toxins build up in their system. To remedy these issues, have your water tested and add biomedia from an established tank.
Read the full article: Understanding Aquarium Filtration Systems
