I know Chris recommends foam, but other tank builders do not. I would definitely use marine grade plywood, though. Salesman for our big tank said no foam, put the tank directly on the plywood. It was a conundrum since we had to use foam under the 300 gallon tank.. we went ahead and put foam under the big tankYes, "rimless" glass tanks for sure (that's what I had previously). The builder for my acrylic (Midwest Customs) advised I needed to use (the pink) foam. I figured if the builder was telling me to do that, then I didn't want to question. I ended up using 1/2" R-Tech insulation foam and he said that should be fine. I find that 1/2" has enough flex and density.