Modern Freshwater Fishkeeping

Fish generate ammonia in the aquarium, and it should be removed straight away. Not even large water changes are sufficient. Ammonia in high concentrations will damage the fish very quickly.

So when a filter sponge is full of brown looking dirt, it's not really fish waste, rather a colony of aerobic bacteria that will convert the ammonia to nitrites, and nitrites to nitrates. This happens when water is forced through the sponge causing new aerobic bacteria to grow. More bacteria in the sponge means more ammonia can be processed more quickly.

The nitrates in the aquarium water that are left over at the end of the nitrogen cycle are not as harmful as ammonia, and don't have to be addressed immediately as ammonia does. However the concentration of nitrates in the water will increase to harmful levels over time and partial water changes are required to dilute nitrates in the aquarium.

The frequency required for water changes to keep nitrates at a safe level can be established as follows:

1. Decide what % water changes you want to do and do a water change.
2. Each day after that test the water for nitrates. When the tests start to show a reading for nitrates, take the number of days to give a reading in the test and divide by two. That's the required frequency in days for water changes at the specified water change %.