I pretty much have my aquariums under control now, and fully cycled, but see, that's when you can get way-layed by a false sense of security. I know that the majority of the beneficial bacteria that are employed to keep the water amonia free are contained in the filter system because that's where the most concentration of oxygenated water is. The bacteria feeds on the oxygen. (Amonia is created by the fish by - products, and cycling builds up a tolerance to that amonia and a way to break it down by building up beneficial bacteria.)
Changing the filter media is a necessary part of tank care, but it can be tricky if you're not paying attention, as I wasn't the other day having just begun to recover from a new bought of bronchitis. I pulled out the old charcol filter and replaced it. I did the normal soaking of the new pack in retrived aquarium water and putting it in, but I jostled the fiber of the sponge part of the filter and must have released a large number of the bacteria, because right after the change I had a huge spike in Amonia and Nitrites in my tank system. Now I'm stuck having to recycle a tank so it can build up the bacteria only it has a full load of fish in it. I did re-locate a few of them to a different tank for the process but it's much harder with six fish in a ten gallon tank than it is with only one or two. Just goes to show, you can never let your guard down when it comes to swimming with fishes.