Just keep him in warm, clean water, and it should start growing back pretty quickly. Look for signs of white or translucent edges - this indicates new growth.
If you want to move him into the larger tank, you could just lower the water level. That would give him more horizontal swimming room (which they usually love), without causing him to swim far to the surface.
Make sure there's nothing sharp in his tank that he could catch them on. Check decorations for sharp edges, etc.
I definitely would not return a fish because it had ripped or torn fins. They will heal quickly.... The best way to treat this is just to keep him in warm, clean water - and they should start growing back pretty quickly.
Last edited by LittleBlueFishlets; 03-08-2013 at 06:03 PM.