I have had several in the past and they work good. I used to buy a sponge for the (I think...) fluval 404 then chop it in half and use that instead of the cartridge along with the little tray things you can get at online pet stores, filled with bio rings. Never had a problem with water parameters in non overstocked community tanks with it. One thing too is that when you are doing water changes, if you think that it will take a while, you can float the bio-wheels in the tank water until you are done so that they don't dry out and kill the bacteria. Now I run Eheim canisters though because they need less maintenance, work better and are SILENT!