Birch stains terribly. Bad bad bad.3/4 inch is a standard size, pine B/C grade should be ok, but oak or maple 3/4" are usually carried by Home Depot and Lowes. The oak and maple usually have more plys and therefore can be noticeably smoother and flatter than the pine. The 3/4 should provide adequate strength to support the weight, depending on the weight . don't know how your original stand was built, but I would still consider 2 x 4 or 2 x 3 corner posts to which you would attach the plywood, depending on the size of the tank.
edit: I should have said Birch, although some carry maple too. Birch holds paint and stain really well. Oak stains really well