He are a few tips on improving stirling engine efficiency. The stirling cycle otherwise know as a heat engine cycle work on the difference in temperatures but requires no other input to create motion.
It is more than a little weird to see a homemade stirling engine in a fish tank but it is an effective way to increase the difference in temperature that the engine needs to perform work. Probably you may want to ask your fish before putting this in their tank.
Check this one out. How to improve stirling engine efficiency.