I'm building it largely for practice. I have an FX5, AC110 and Mag350 on the tank already. I want to practice, I have two 20s laying around, so it seems like it could work.
A 20 gallon tank properly partitioned will hold roughly twice the bio of an FX5, so it'll be a good addition, at least. Plus, since I'm fairly handy and have all the stuff here, it'll be a lot cheaper than buying another filter.