What type of media are necessary? Depends on your conditions, but personally, I think all you need in most situations is something to perform mechanical filtration -- the foam sponges that come with the ACs do nicely.
Currently, I run two sponges in my AC 300, with some homemade "filter floss" in between. The floss is actually just some polyester batting material I got at a hobby shop.
I clean my media in the way that tstenback mentioned -- swish them around in siphoned-off tank water, mostly. I only change my media when they're obviously breaking down.
One thing that may be worth mentioning: I'm not sure about the AmmoRid product, but the way Activated Carbon works, after a period of time it can leach some of the adsorbed toxins back into the water.