Hi,
I have the same filter you just bought the Aqua Clear. Its a really good filter and is very reliable. What I do is I change my carbon every 2 - 4 weeks. I hardly ever change my sponge, I just give it a good cleaning in old tank water once every month or so, I don't even clean it enough to make all white again just enough to get some of the gunk off. I also change biomax every month or so, sometimes not even that. But I NEVER change them all on the same day there always changed a good one to two weeks apart. This routine has worked for me very well. This is what I personally do, its just to give you ideas. You really have to do what is best for your tank.
For example, I have two filters on all my tanks for a back up, extra filtration, and an extra supply of bacteria when the other is rebuilding from when I change the media. Some my say that's not even necessarily.
Good luck, it seems like you really trying with you fish : )
TigerLotus