process
Biological filtration
The filter's real job is to house bacteria, not to trap dirt. Media choice, flow rate and oxygen supply all matter because the organisms need surface area, contact with the water and oxygen. Almost every 'my filter isn't working' problem is a biological one rather than a mechanical one.
In one sentence
Biological filtration is the microbial oxidation of ammonia and nitrite on surfaces inside the filter. It is what makes a closed aquarium possible.
What actually determines capacity
- Surface area of the media, weighted by how much water actually passes through it rather than around it.
- Oxygen supply, which is why a filter starved of flow loses capacity.
- The ammonia supply itself — the population sizes itself to the load, over weeks.
- Temperature and pH. Nitrification slows markedly below about pH 6 and at low temperatures.
Maintaining it without destroying it
Rinse media in a bowl of tank water, squeeze sponges rather than scrubbing them, and never replace everything at once. Manufacturers' instructions to replace cartridges monthly exist to sell cartridges; following them keeps a filter permanently immature.