A Rena XP3 came with the aquarium, but after spending a whole bunch of time reading posts on the Planted Tank forum, I believed it was not going to be enough for the fish load I wanted to keep. Since I already had a Rena, along with a variety of parts and so forth, I bought an XP4. A friend of mine showed me his FX5 (looked like a small wet/dry vac!), but I don't think it would have fit inside my tank stand. Also, since I have two canister filters, I can clean them on alternating schedules to minimize disruption to the biological filtration that they are providing. For the investment I've made so far, I like having the redundancy of two filters, just in case something was to happen to one of them.