Many people forget about the nitrogen cycle when they have been keeping fish for some time. It can catch people unaware.
Quite often, people might upgrade their filters. Either by changing the filter material with something with more surface area (e.g. from filter sponge to ceramics), or by replacing the entire unit with a larger or more powerful one. This can put your fish at risk. Why?
The new filter will be devoid of the beneficial bacteria that converts your fish wastes to safer substances. You will be in danger of “new tank syndrome” whereby you can get dangerous levels of ammonia or nitrite that can kill fish. Why is this?
The majority of nitrifying bacteria do not live in the water column. They pretty much only colonise the surfaces of objects. This means the surfaces of the glass, plants, gravel, rocks, furnishings, etc. It is the filter that harbours the greatest number of these good bacteria because the filter material is socially designed to have a large surface area. So, by removing the old filter or filter material, you’re effective getting rid of your good bacterial population.
So how do you avoid such a situation? The best thing to do is to seed your new filter with old filter media, or run both filters at the same time, while the bacteria has a chance to establish themselves in your new filter. This might take up to 4 weeks in freshwater tanks, and up to 6-8 weeks in marines.