Powerheads. I'm digging the vortechs. They aren't cheap, but if they do what they claim, I'm ok with it. I'm sure some of you actually have these. How many would I need for the tank if I do use in-tank powerheads, which I probably will now that I've checked these out--2? 3? 4? Would it be better to use 2 of the next size down on each side or one of the 60's on each side? Seems like the 60's may blast over a 20lb rock...