As an alternate but one I don't "really" favor too much, one can also look at shimming the tank. One can do okay with shims but it does take some thought and it can also gradually let you down over the long term, but still better than trusting filling the tank and simply hoping it doesn't go fatally bad!
Shims do best when the top of them is level as it doesn't create the stress on the trim/glass that we get when a single shim is used which leaves the glass setting on a sloping shim. To get a totally flat top surface on shims, we need to use two at all places by placing the narrow portions together and then sliding them together to raise the amount shimmed. That may sound nuts and difficult but when you look at a pair of shims, it will be easy to see how to get a level top on them. The reason being that the glass gets much more support from a flat surface and that makes it farless likely to cut into the shim and gradually lose the support we want.
Overall, I do not like shims as well as getting two flat surfaces together but there are times when we have accept a "less than perfect" solution. My problem with adding spongy material is the amount of pressure a 75 puts on that spongy stuff. I don't feel like we get true support but more of a feel good item as the 75 can just simply push the support out of the way? I also find that I drop so much water on the spongy that it creates a problem for the underlying wood as it takes so long to dry when I can't get to it to wipe up the spill. It can look better, but then if it damages the wood over time, I'm not happy there, for sure.