I recently made an upgrade for my p.bass and built him a 750g pond outside. When I moved him in he had completely normal coloring for a few days but now he has turned almost completely black with only a few spots towards his tail end remaining. The water quality is pristine (0 ammonia 0 nitrate 0 nitrite) and he is acting completely normal. The pond has no substrate and the liner is black so maybe that is whats causing it? Does anyone have any suggestions?