Are you putting a glass or acrylic aquarium on it? For a glass aquarium you need to support the perimeter. For an acrylic aquarium you need to support the entire bottom.
M1A1 is correct, tweaking the aquarium is very detrimental to its structural integrity!
For an acrylic aquarium I would use a piece of sealed 3/4" plywood with a piece of closed cell foam on it. Yoga mats are a popular choice for closed cell foam. I am using a 1/2" thick camping mat from Walmart (~$20) and just put my 180b aquarium directly on top of the mat. (Yoga mats were not long enough).
For a glass tank all the weight is supported by the perimeter so you will want some VERY dense foam or it will just flatten and not provider and leveling support.
(A picture of the stand would be very helpful!)
Most steel stands are going to project the contour of the floor to the top of the stand once the weight of the aquarium and the water is put on them. So if the floor is un-level the stand will just flex to the contour of the floor. You can use this flexibility to your favor by putting a few mm shim under the leg on the low corner to level it.
If you have a heavy duty boxed frame stand it is less likely to flex and a different approach will be needed. I.e. you need to provide more information if you want informed help.
