I wouldn't put this on a 3/4" shelf. This tank, assuming you'll have sand and LR
in it, will weigh around 1200 lbs if not more. Also, since this is only a 4' tank, you might want to check the floor joist orientation under the tank. If the joists are perpendicular to the tank, you'll probably be OK there. If they are parallel to the tank, the entire weight of the tank will be resting on no more than two joists and the floor will probably need extra bracing...assuming you have joists. If you're on concrete, you're in good shape there. Since I'm not a structural engineer, I hesitate to advise you on how to brace the shelf. I'd remove the shelf and put in an aquarium stand that is designed to handle the tank. My 110 caused the joists to sag a bit and I had to go under the house, double the joists, and add floor jacks...my house has 2x10 joists.