Supporting a bookshelf aquarium
In addition to your supports maybe place a 72" piece of 2-inch structural square steel tube, sort of like a closet pole for hanging all your lead filled jackets.
Mount the pole (tube) about 8" forward from the back wall, right up against the bottom of the OSB/plywood shelf. The end pieces could be 2"x12" quarter inch plate with a welded square nub in the middle. Multiple lagbolts go into the side walls.
If the filled tank causes shelf to curve slightly, drive shims between tube and plywood. You could also beef up the plywood and eliminate some of the wall brackets.