i like my emperor 400. for freshwater it is great. it comes with 2 biowheels that culture bacteria. once its fully gunked up, you can float the wheel in your tank for a few minutes and all the fish will devour the nitrosomanas (spelled wrong). it has plenty of filter space, and you dont have to just use carbon. anything will work as long as it fits in the cartridge, and if it doesnt fit, just take the cartridge out and put it right in the container! i used to put chemi pure in one side with filter floss and the other had carbon in the cartridges and more filter floss.