This was him yesterday and this is him now. Just added myxazin, which treats fin and body rot, ulcers and bacterial infections. The top of his back/middle fin has also gone grey. I did a water change before adding him yesterday so water should be good but I'll test it in a bit. Only other inhabitants are shrimp and two snails. I Also added stress coat.

Anyone got any idea what it is? It doesn't look red. He doesn't seem especially active but I don't know his temperament yet. I introduced him slowly to the tank and left the lights off for a while so tried to make it a stress free introduction.

Edit: water test results are all fine as follows:

Temperature 22 degrees Celsius (may need to be higher)

pH 7.4

Nitrite 0ppm

Ammonia 0ppm

Nitrate 0-5ppm

Edit 2: I've increased the temperature to 25 degrees Celsius.

Edit 3: I've added aquarium salt in the stated dose.

Edit 4: He died in the night. Looks like it was graphite disease as he was much more grey this morning. Not much more I could have done unfortunately. Thanks everyone for your help and letting me know it could be graphite disease so I could mentally prepare. He's going back to the fish shop today. I doubt it's their fault but should be able to get my money back and they need to know their stock might have it.

Picture of him dead for future reference.