Have any of you guys had to change the filter yet? I don't want to have to special order the filter cartridges online (I didn't buy extras with the tank). So I picked these up at Petsmart -- Marina i25 filter cartridges. They are not a perfect fit, but they fit well enough. They are about $5 for two of them, if I remember correctly. Just in case some of you are in the same boat as me!