How long can cooked fish be left out in a container?

I would not recommend cooking and consuming fish that have been left out overnight. Whether or not it can be cooked safe would depend upon variables that could include the type of fish, the packaging (if any), previous handling, the actual ambient temperature, the temp/time exposure of the fish itself, etc.

There are certain types of seafood (tuna, mahi mahi, bluefish, sardines, mackerel, amberjack, abalone, etc.) that can form histamines if left at temperatures over 70 degrees Fahrenheit for longer than 4 hours. The histamines cannot be cooked out, can cause illness, and cannot be detected without chemical analysis.

=========================================

A Different Opinion

If it was not previously refrigerated, probably. Fresh fish should, however, be cooked promptly.