I would recommend 3/4 pressure treated with a laminate glued to the surface. If the tank is sitting away from the edge, double the sheets and add a banding. Pocket screws and heavy duty construction glue will work. I have found that salt water tends to eat the glue over time in some untreated...
I wouldn't do it. Water spills will ruin the plywood sub floor. If you have a major leak, the water will go through the floor into the downstairs ceiling. It happened to me. A tank seam went in the middle of the night and the tenant on the first floor had 20 gallons of water come through her...