Hi,
Welcome, even though you joined a while back. Thanks for posting. Like mentioned, get the stand leveled. The mat or foam is generally used for rimless or frameless tanks. The issue with putting anything under a framed tank is the chance it puts pressure on the bottom glass and not just the frame. The bottom glass is not meant to support weight, so it may crack.
I have a Marineland 180 framed tank. I built my stand. If you get to know me, the frame could double as an old school seesaw. :p Anyway, after I built the stand I made sure it was as level as possible off the floor. I then purchased blue solid insulation foam from Lowe’s as a leveling pad. The important thing is to be sure if it compresses 100% under the frame area, the blue will still not hit the bottom frame glass.
BUT, if you get you stand shimmed and leveled you should not need that. I did use a furniture grade piece of plywood on the top of my frame also.
Hope I made sense . Let’s see some pics when you can.