the stand looks great! having just made one I can completely relate to how tricky it is making all the angles "true"! water is amazingly dense/heavy and can cause settling in a stand and even sometimes the floor underneath it. I would highly recommend getting this sorted before moving forward as it's much harder to have to take the tank down just to fix a stand issue, BTDT. I would take a square and make sure the main supports are still still at right/90 degree angles. I would make triple sure that the level issue isn't being caused by something in the stand giving out. cross bracing is quite important when something is top heavy. you can hide cross bracing in the back or inside... it will do wonders for reducing flexing in the joints and will make the structure much more stable. Metal brackets can help a lot if you don't have much room but need lots of strength! I've done quite a bit of general building, I wouldn't say I'm an expert but I do speak from general experience on this one. also, a few extra well placed screws in the siding, fastening the siding more to the internal frame can add quite a bit to the lateral stability. you really don't want any of the load bearing parts of the stand to fail with the tank full. IMHO
, make sure the water level change isn't coming from fatigue in the stand when being filled. if it's the floor, then shimming is your best bet. shimming can also take care of issues where the stand isn't perfectly square (which is very hard to do) but otherwise structurally safe/sound.