That particular model is now the aquaclear 20. I have it on one of mt 10 gallon tanks. There are three different filters in it. A sponge, which should rest at the botton of the right chamber. A second/middle layer of activated carbon, and a third/top layer of white biomax "balls". Do you have all three filter layers? If not you can go to you LFS
and buy the replacement cartidges for the model. It is a great system. You would have wanted to replace those parts of the filtration anyways. Also, you may want to replace your sponge filter if it wasn't used in a while. It may have grown some yucky stuff or not been stored in a safe way for your new setup. All the replacement parts are relatively inexpensive. Should not cost you more than $6-8.