Join Date: Apr 2010
Location: Near Boston, Massachusetts
People more knowledgeable than I have already replied, but I'll put in my penny's worth anyway.
This stand may be fine, or it may not: the only way to know is to wait. Here's what I recommend. You've already invested the time and money into building it, so don't just throw that away. Find a safe place (ideally somewhere the same temperature and humidity as the room where it will finally go) and set this aside for a couple weeks. In my experience, two weeks in a dry environment is enough to tell you which boards are liable to warp. In this case, it may be all of them, it may be none of them, or, most likely, it may be some of them. I've seen pressure treated lumber bow and twist, so I'd be a little concerned about just setting up the tank. A twist in the verticals will be a cosmetic issue, but not much more than that. A twist in the long horizontal members could be catastrophic if there's a tank sitting on top of them.
If you were going to re-build, given the weight, I would go for either dry solid wood (construction lumber, set out to dry long enough that you know it's warped as much as it's going to) or a metal frame, preferably professionally welded. Water weighs a LOT, as you know, and the stand really needs to be solid. The design you have is good vertically: if you were to screw a plywood face to the back and sides (1/4" would probably be fine) I'd stop worrying about horizontal strength as well. I did that to an old workbench, and it made an enormous difference.