I will probably be purchasing one of these within the next few days since research seems to prove they're the best for what I want. I'm going to have a 30 gallon tank with 2 or 3 goldfish in it.

What I need to know is how do the filters work? With most filters, you just buy the little carbon/foam inserts and you're done. I am under the impression that this one is biological so there will be more to it. Can someone please let me know what types of inserts I'll need to buy for it, and what they're going to run me (on average)? I'm trying to do a lot of ordering online so as to get the cheapest price, and I need to know exactly what I'm looking for.

I'd also like to know what each medium does. If anyone has any good links, I'll be more than happy to read up on the info myself. When I do a search for anything related to the AC110, all I get is sites trying to sell me the filter, and not explaining how or why it works.

Thanks.