Lots of animals go a little paler if kept in the dark for a long time. If left I the dark for lots of generations, like animals theatre live in dark caves or the deep ocean, then they often evolve to be white. This is because it costs energy to make dark colored skin and so if you don’t have to do it because there is no sunlight then you don’t bother. The benefits of having a dark skin means you are protected from strong sunlight and it’s a balance between the two.
I agree with Andrew and Aggelos. It also depends on what colour the fish was originally – it would probably turn paler after a while, and maybe it could go white during its lifetime if it was already a pale colour.
It depends a bit on the kind of fish. One of the coolest things I’ve heard about fish is that there are some that are coloured darker on their top half than on their bottom half. This way, when sunlight shines through the water, their colour evens out – it works to make it harder for predators to spot them! (Otherwise, their top half would be illuminated and brighter because of the sun).