If I would you I would do is
1. Set your temp higher to 82f (piranha are active with high temp)
2. 29gallon are to small and it will stunt it grow, due to Serra is a slow grow anyways.
3. Remove some to all decoration for him to have room to swim.
4. Most piranha doesn't like bright light, I would dimmed your light.
5. Don't feed him every day, every other day is good.