Just looked at your pics. I thought you said you had fancy guppies, but the pic of the fish looks just like my otocinclus did when I put them in a bucket during big cleans. When I leave them in there too long they lose all their color and turn translucent like yours. I usually figure that this is because they don't have anything to suck on, so now I throw a decoration or a rock from the tank in with them in the bucket. They stick to that and their color stays the same. I don't know if that helps, but I figured I would throw it out there.