" What about immerge the "bucket of water being heated " in the main tank with the fish?"
In your case, the "bucket" is the pond.
If you literally put the "bucket" in the pond, it would be indirect heating (closed circuit, less efficient) and would be better accomplished with a coil immersed in the pond or sump (better) instead of the "bucket".
Size of the collector depends on the amount of sun it gets and it's efficiency.