Holy moly thank god you didn't fill that. The tank rim must sit on a flat surface. Only level the stand like everyone said. I also use composite shims from lowes, that way if they get wet they don't break down. Also, definitely use shims not something else, they provide the most adjustment and ease of adjustment. They are meant for that purpose and super heavy loads.
I personally shim the stand level, then put the tank on and adjust if necessary, then fill the tank a quarter and adjust carefully, then fill it half way and adjust even more carefully one last time. I do not recommended trying to adjust shims once the tank is more than half full. Also, make sure you fill in all the gaps with shims after leveling.
For example, if you need one on the front left corner, you will need to space a few out on the left side and front left (pushed less and less in as you get further to the back and right). Does that make sense? I like to keep the gap between shims 6-8" max until you can't put another in.
One last thing, don't knock the secondary shims too far in because then you will have to adjust the rest again and end up leveling until your tank is 4 ft off of the ground lol.
Good luck!
