My Chaetostoma plec (bulldog,wide-mouth, insert name du jour here) won't eat algae tabs at all... He's doing well, grazing on whatever food the rainbows don't get and what little algae is in the tank (it's the only algae/scavanger type in the tank). What exactly do you mean by lighter -- recently, since you introduced it into the tank, etc? Mine changes colors sometimes depending where it is in the tank as if it were trying to blend in with the gravel, sort of light mottled splotches. What color is your gravel? Have you tried blanched lettuce or zucchini?