Not much help right now, but when I'm building a stand, I generally use the kiln-dried 2x4s or 2x3s. It's slightly more expensive, but generally not that significant a difference when I total up everything for the aquarium, or even just the stand by itself.
Despite being kiln "dried", I've found some that are practically sopping wet... So I just go through until I find a couple pieces that aren't twisted, are straight, and have a grain I like.
And then I plan the pieces out to minimize how the warps and knots will affect them- I'm fine with knots on uprights, but try to keep them out of the middle of horizontals.
For the finished stand, I'd check the top using a straight edge or something. You might even be able to just eyeball it, and take down the highest corner/corners (or middle, but don't worry if the middle sags). Maybe a sander/sanding disk if it's not much, if it's a lot, a rasp or a plane to even things out. Just shim the bottom once you make sure the top is even.