How can I build a shelf to support a 15 gallon aquarium?
Using the Sagulator, you're borderline for a noticeable sag... It will also depend on how deep your shelves are, and how wide the aquarium is (I assumed 16 inches deep shelves). Since you're using plywood you're going to be facing the shelves with something, right? I would just double up that shelf (two layers of plywood glued together) and you should be fine. 

Also, you can run a strip of the plywood along the bottom back edge of the shelf which, combined with the front facing, should support all the weight you can throw at it.