Sure. I know an easy, simple, compact design for multiple tiered tanks Sorry that I just assumed! I haven't had a good night sleep in awhile, sorry that I was a bit cranky. lol
2x4s would work perfectly. What size tanks are you going with?
Also, I highly suggest putting your frag tank up top, where water from your QT tanks can't leak/drip into it accidently when you put your hands in the tank etc. It's still a bit risky, IMHO.