Yeah it could be done that way. Not the best choice obviously, but do-able. Not too sure how well they hold up or how secure they feel, but I've seen it done.
Definitely wouldn't use plywood (unless done how GraphicGr8s mentioned), as I think they are not as strong and more prone to warping. I think 2x4's would suffice (not that much weight really). I would make a rectangle form just to help provide some sort of support/weight displacement on all sides of the tanks (might not help much, but hey, why not?) rather than just the front and back. Would advise using screws instead of nails (try to not split the wood, pilot drill holes first if necessary)