Even 3/4" plywood will warp over time. 48x18 footprint is a LOT of water, even if it's shallow. I wouldn't.
However, if you set the tank on the stand so the front is flush, you could built a 2x4 stud wall to go under the back overhang. Full length 2x4s top and bottom with a stud every 24" going vertical would be enough support. Just screw/clamp it to the back of the stand.