Yes, I think the concrete floor is uneven from the get go but not by much. We had to put many shims under multiple feet to get it very close to level.I agree, get this thing level; as well as braced.
Is the issue with level where the stand rests on the floor?
Here is what I can tell you. Before filling the tank the back length wise was perfectly level. The front lengthwise was very slightly lower on the left side so close that the bubble on the level was not touching the line. The front to back width wise seems to be consistently very slightly lower in the back over all points evenly.
Now that the tank is full, the back length wise is slightly lower to the left and all other measurements have remained the same.
All the measurements of the level ( 5 foot level ) are so slight that the bubble never touches the line.