I would bet that a smell/taste test would be your ultimate
indicator regarding whether the fish is edible or not.
The temperature of your refrigerator plays a big factor.
At my highest setting juices and liquids start to freeze. I keep
the fridge at the temperature right below where this happens.
If you add preservatives like vinegar, lemon juice, and
salt the fish (or practically any food) can last a long time.
Many people use these ingredients to ferment foods and
they last a ridiculous amount of time.