That might be correct, but it might not.
What tests did he really do to see what fungal
agent was involved?
If he made a diagnosis before doing a necropsy,
that seems a little strange, and not in tune with the
scientific method.

I'm sorry that I'm skeptical, but I highly doubt
your hands in the tank caused this problem...

This could have been fungal or bacterial,
if he didn't culture a sample, he really can't know for sure.