^^ this is what i gathered after reading the thread a couple times:
A fish tank is a small biosphere. To make sure that this bioshere is healthy you need to maintain balence.
1.) In order to control the waste that the fish excretes, you need to have a balance of ammonia eating bacteria. This colony needs to be established before introducing fish or the ammonia levels will be too high in concentration for the fish to live and basically the fish die in their own wastes.
In order to establish said colony, a 5ppm level of ammonia is suggested. It seems to be a concentration that characterizes an acceptable level of fish waste in a tank. When you have a colony capable of processing a 5ppm level, you are ready for the next step.
2.) Said ammonia eating bacteria also produces wastes. Their waste is nitrite(NO2-). As more air gets introduced, Nitrite forms nitrate (NO2- to NO3-). In order to control nitrate levels you need to change the water.
I gather that the concentration of nitrate either a.) has a higher concentration threshold for the fish to survive or b.) the rate of generation of nitrate is a lot lower than the rate of generation of ammonia, therefore its more efficient to "condition" the water rather than change it whenever the ammonia level are too high (expensive for 50+ gallon tanks).
Please correct me if im wrong, its the best way for me to learn.