To add onto what Albert said, we have had members in the past who bought safe species of fish from the grocery store to feed and ended up with dead snakes due to a preservative they didn't know was in them. Similarly you have to worry about contamination/pollution in wild caught fish. It's just something you need to be really careful about if you do it.

IMO fish should be an option to have on hand for when you have snakes that are refusing all other foods.
I currently have a tank of feeder guppies in case I end up with any non-eating young ones this year (I had a few feeders left alive after getting my nearly ftt baby eating last year and couldn't bring myself to just flush them. Now I have about fifty).