I've done all the environmental research, purchased the aerator and heater. What should I use the money for next?

I think the Magic Plants are very useful once I have Fish Environment researched.

The potion is fun; if it doesn't work once try again. If you look closely at what fish are in the tank at the time, you'll see their fins will switch around, or you'll get a new fin. I had played many hours without a tiger fin and got one from the potion.