ok i have some questions...maybe some prety dumb ones...anyhow. inside my filter unit i have 2 filters one is that blue kind and the other is some kinda corse black material. now the guide that came with the tank says you should change the filter every 2-4 weeks...are they talking about the blue one? they sel them at walmart for pretty cheap. the reason i ask is that i have read that changeing to a new filter will get rid of your bacteria and create a new cycle....but isnt it true that if that filter gets nasty as all get out it will cause messed up water quality? please hlep me understand so my fish can be happy again!