3/4" plywood alone is plenty if it's done right. With a glass tank, all the weight is supported by the perimeter, there is no need to support the bottom pane at all in most cases. The tank needs to sit directly over the plywood and be supported all the way around.
My 90 is on a stand that is made entirely of 3/4" plywood with 1x3 pine around the top and bottom perimeter. The pine on the bottom is strictly decorative and the top surronds the tank, there is no weight on it except above the doors where there is a strip of plywood glued and screwed to it to support that much of the tank. Everything is glueed and screwed. I don't know how old the stand was when I got it but I suspect it had been in use with the tank since the tank was new (20 years ago). It was only screwed together when I got it, I toook it apart and added glue before reassembling it and refinishing the stand. If it can hold my 90, it will have no problem holding a 75. my stand also has no floor, the bottom is open. The whole thing is made of 11 pieces of plywood and 8 pieces of 1x3. The back and sides are single sheets and the front consists of 3 upright pieces (ends and center support), 2 strips (above the doors), 2 doors anda piece of 1/2" at the top, which the tank rests on. There are 4 pieces of 1x3 at the top and another 3 pieces at the bottom. I'm not sure that the 1/2" plywood top is needed and the rest of it could have been made from a single sheet of 3/4" plywood.