i agree about the fish water, that might not be a good things, and ive kept fish for a while cichlids and salt water tanks primarly, and im just getting into the snake keeping business and drinking fish water might not be that great, theres elevated levels of toxins in the water such as nitrites, nitrates, possibly even amonia if the tank isnt filtered well, also if its a real slow filter/ water running, the water could contain diseases, even ones brought home from contaminated fish from the pet store...

but about the OP a BP eating fish would be a sight to see, but i dont think it could happen, i mean it COULD but i have my doubts...and if it was, would fish be healthy for the snake?