I'm about to set up two ten gallon tanks with air-powered sponge filters, and while I'll probably go the DIY route, I was shopping on Amazon anyways and came across the Hydro series of sponge filters. They go all the way from I to VI I think, but I was wondering what the difference was. Is it just the size of the sponge? I realize that more sponge = more bacteria, but is there like a cut off to how big the sponge should be before you're not mechanically filtering with the air lift anymore? These ones on Amazon were pretty cheap, and I'm sure if I ordered them through my LFS they'd be even cheaper without shipping. What's the consensus on DIY vs just buying one on reliability and price?