Agree.....I've also studied genetics a bit (I know....boring). This is entirely possible. Not common, but possible. Most of us have heard of albino deer or turkey....why not fish. This certainly isn't albino, but it could be amelanistic or hypo melanistic.
Yeah that could be it maybe fish that live under cover their entire life, I was also thinking maybe food supply? Different nutrients in the water...or the fact they were pulled out of a "Water Reservoir" next to a Dump site lol