but I'm no discus pro. The discus in one of my tanks are always dark, from time to time they will lighten to normal color then redarken(except for my golden who is always golden lol). They range from 1.5" to 2.5". They eat quite well, are active, and do not show any other signs of illness. My water hardness is about 155, ph steady 6.6, temp 83, ammonia nitrates etc 0. Tank has been up for quite awhile now and they have just always been like this. They are more skittish than my older discus so I thought perhaps its just stress. If so I don't know quite what to do to fix that. Any suggestions are of course greatly appreciated! I love this site