Stainless steel shim stock would be the best (can be found a local hardware or amazon, won't be affected by saltwater/weight). Composite plastic/wood could compress/condense over time. Most important to shim the corners where the pressure points are in your setup. I like the open span you have on the front of your stand. How was that done? Usually there is a center support for that kind of span.
Thanks everyone! Well I bought the stand with the tank and that's how the guy had it but I had to redo a lot and added some extra braces bc it didn't seem too sturdy before...the front has 4 supports but I couldn't put one right in the center because I wouldn't be able to get all of the equipment in. This was the stand before I redesigned it how it is now