How large is the tank? You really want to make sure every piece of the stand that is contact with the floor stays contacted to the floor via whatever you use to elevate the stand. Meaning if the stand is engineered to have the entire perimeter touch the ground, putting a couple 2x4s under the outside edges to have a "gap" under tank is asking for trouble.
You could also use those drainage tiles they use in wet areas like restaurants and what not. Here's an example
Drainage tiles
I'm sure you could find them cheaper elsewhere but this could be a good first step.