I am new to Indigos and have read that many owners feed fish. This reminded me of when I ranched mink in the late 70's. Certain freshwater fish(forgot which ones) were avoided in the feed mix due to possibility of thiamase poisoning. I do not know if this could apply to snakes. This is from the first site that came up when I googled thiamase:
Goldfish and minnows as a straight diet can be bad because their flesh contains the enzyme thiamase, which breaks down and depleats the B vitamin thiamines.
RP
