Fresh Water Tank Sagging

We had an object kick up and damage our drain plug that started a small leak at the thread to tank location. That small leak ran down under the tank and onto the 3/8" plywood, that was supported by metal straps, and when wet started the sag below the tank.
I drained the tank and when I got home I fixed the leak, and installed 3/4" plywood under the tank and reapplied the straps. 3/8" plywood was too weak to properly distribute the weight to the straps once it got wet.