Biological filtration uses microorganisms on filter media and other surfaces to convert ammonia to nitrite and then nitrate. To keep it working, preserve established media and provide suitable water flow and oxygen. Replacing a dirty-looking sponge can remove biological capacity the fish depend on.
Separate the three filter jobs
- Mechanical filtration traps particles such as food and debris. Sponges and filter floss commonly do this.
- Biological filtration processes ammonia and nitrite. A sponge can provide this function as well as trapping waste.
- Chemical filtration uses a material chosen to adsorb or otherwise remove particular dissolved substances.
One filter can perform all three jobs, but one medium does not automatically replace all the others. Clear water is not proof that ammonia is being processed.
Give the biofilter time and a suitable load
New media does not have the same capacity as established media. Cycle a new aquarium before stocking, then add animals gradually. A sudden increase in feeding or fish can exceed the filter’s current capacity.
Use our fishless cycling guide for a new setup. A bottled starter may help establish a system, but tests still need to show that ammonia and nitrite are being processed.
Choose media by function and maintenance
Reusable sponges, ceramic media, and other purpose-made biological media offer surfaces for colonization. Choose material suited to the filter design and keep water flowing through it. Packed media that becomes clogged cannot do its job well.
Do not expect standard biological media to remove nitrate completely. Nitrate control still needs a plan appropriate to the system, such as water changes and plant uptake. Our nitrogen-cycle guide explains the sequence.
Clean without throwing away the colony
When flow falls, check the intake and trapped debris first. Gently rinse reusable biological media in removed tank water. Keep it wet during servicing and avoid replacing all established media at once.
Follow the instructions for disposable mechanical pads and chemical media separately. Replacing exhausted carbon does not mean the whole filter needs new biological media.
Investigate a loss of capacity
After a power failure, drying of media, major filter cleaning, or a treatment that may affect filtration, test ammonia and nitrite. Check oxygenation, flow, temperature, pH, and buffering rather than assuming a fresh bottle of bacteria is the only response needed.
UF/IFAS explains the biofilter’s oxygen and alkalinity requirements. Our new-tank syndrome guide covers a waste load the biofilter cannot yet handle.
