...and for supermarket fish, one day as it's already been hanging out awhile.

Two days is fine, but three is the threshold.

You've heard the expression, "Fish and visitors stink after 3 days?" 2 days max.

Depends how fresh it was when you got it - I would not go more that 2 days though and I would give it a sniff - if it smells fishy it has to go - when in doubt throw it out!