+1 to DogFish's reccomendation. No need to buy anything on an ongoing basis. The foam will last and last and is all you will ever need. It does not loose its bacteria if rinsed, as instructed, in tank water. Better yet, once it gets a bit older the bacteria will develop a very sticky nature and will polish your water as good as anything can. Superclean.
Definatly going to look for some at the pet store this would help our budget so much. the girls are wanting to add some tank mates.