Fish tank water cycling helps maintain and establish a proper nitrogen cycle, which is important to keep your fish healthy. This is especially important when adding new fish to your tank to prevent issues like new tank syndrome.,Cycling a freshwater aquarium refers to the process of allowing beneficial bacteria that break down waste to grow in the filter, water column, and substrate. This is required to prevent fish from living through a dangerous period where ammonia and nitrite levels peak and then fall, which can lead to health problems. This tank cycling should be carefully done before fish are added to a fish tank. Fortunately, there are products available that enable cycling with fish already in a tank, and these products also speed up the process.