Sick Discus? (Pics)

[Julianhossini]

Hi i recently got 2 young discus and the smaller of the 2 seems to be staying in the corner by the heater all day and also doesnt eat. when i got him he was very pale and but now he seems to be getting very dark? Is he sick? Or very stressed? Does he just need to get used to the tank? Do i need to worry? Plase help, thank you
Fyi, the first 2 pics are of before and the last 3 are of his color now