I have five different kinds of discus in my 50 gal. tank for almost a year. I have a blue diamond, blue snakeskin, a marlboro red, a golden sunrise and a snow white. They almost all the same sizes. All look happy, healthy, eat well and almost all same sizes about 4 inches in diameter. The marlboro red, golden sunrise and the snow white when I bought them looked really good, but after a few months in the tank they started developing darker dorsal, caudal and tail fins. The marlboro is worse because the black pigmentation extends to the head. The other two doesn’t seem to be affected and all seem to be healthy. Except for the color change I am very happy with them as a matter of fact the marlboro and snakeskin paired up and bred but eggs didn’t hatch. By the way I also have 7 cardinal tetras and 4 rummy nose in the tank.
It is caused by the strains and the pigmentation.
The strain has not been stabled with cleanness of the color.
And it has been cleaned temporary for certain purpose.
It will come back to original color in certain length of period or stressed will turn dark in color.
The Marlboro Red comes from Pigeon strain. Pigeon discus has dark pigment.
Many breeders have successfully cleaned the black pigment from the pigeons.
(View clcdiscus: Checkerboard Pigeon, Melon. . .)
The pair of Marlboro and Snakeskin you have is not advisable to breed. It will give you a lot of disappointment.
Look forward. Put yourself in the front line. Get a clean Marlboro to breed.