DD1,
Some of the experts on here can answer your question with more accurate scientific facts, but in plain Texan, fish eat and produce nitrogen-rich waste material(FS)and ammonia is a product of that process. Specialized, neat microorganisms breakdown this ammonia and convert it first to nitrites and then nitrates. Biological filtration is a wonderful process which provides a home(the filter) and an optimum environment(the materials in the filter), for those microorganisms to live, multiply and do their thing, which is consume nitrites and nitrates that will otherwise harm or even kill your fish.
Getting seasoned is simply my way of saying getting the microorganisms established in the filter in sufficient quantity to remove all the ammonia. It has taken a couple of weeks to get there...sometimes it can take even longer, but what a wonderful process Someone designed to make use of everything, nothing wasted.
