Use a level for all parts of the stand first, be sure to check front to back as well as side to side that everything is level before placing the aquarium on top.
Once the aquarium is placed on top check to make sure that everything is level once again.
You may need to adjust using the feet to account for the flexion in the stand. Be sure that all parts of the stand are evenly supporting the aquarium leaving no spaces/gaps.
SQUARING: the aquariums DO fit square to the stand, if yours is not entirely square please perform the following Start by getting the front corners squared up which will show it is the same length then if the back is out look at the feet to make sure they are all set properly.
The top rear also has some movement because the cross bars under the plumbing that squares that panel up are positioned half-way down the frame. If the tank is on the cabinet one or both are out you can simply reposition them by hand. But ultimately you will need to make sure that it's not resolved by adjustment of the feet first.