Fresh fish on Passover

Can one buy fresh fish in a non-kosher store on Passover, such as Fairways or Costco.
Any difference between salmon that is sliced, or whole fish on the bone?

In the big chain supermarkets, where it’s not worth it for them to try to fool you and give you the wrong fish, you can rely on what they label the fish to be.
A private fish store, on the other hand, the owner might try to fool you by giving you a cheaper fish and saying it is a kosher type. The solution there is to buy the fish with some of the scales still attached. Or just order them whole and have them filet it in front of you.

Is it ok to buy fish from Fairway or Costco during Pesach given that their equipment will is not K or KP, but it is cold?

Yes. No problem

Hi. If by mistake such fish (from a supermarket) was not rinsed at home before baking, is it ok after the fact? (I am asking about during the year, rather than during Pesach). Thank you.

It’s ok