To go from a 15 Gall. to a say..20 or 25 Gall,....isn't that about $40? You could still get away with the same filter system (I assume), and outside of possibly an extra stand (Some small table that can hold 200 lbs., and is level..) If you're going to spend the money on the fish,..why stick with 15 Gallons, and try to justify conditions that shouldn't be? Go big and go nuts! Just do a bit of research before!
Good Luck with it all.