Once your fish has thawed completely it will last about two days in the fridge before it starts to spoil. But this i snot always true as the length of time it stays fresh is often dependent on the variety of fish it may last longer or shorter.u003cbr/u003eu003cbr/u003eSo, bass, tuna, salmon, flounder, and trout can last between three and five days in the refrigerator. But if you are storing oily fish like mackerel or sardines you may have a much shorter window before the fish starts to go off.