Angel fish discolouration

Question
Hi there, wondering if u can help me as we are confused as to what is causing our angel fish to loose colouration or more like a change from a shimmering to a velvety look.I believe they are black marble angel fish which we have 3 of.The water tests are carried out every week and are all ok.None of our other fish, which include congo tetras, rosey tetras,golden rams and a few species of corydoras as well as 3 species of plecs.Water temp is 28'C PH 7.4, Amonia 0, nitrates 40, nitrite 0.Any ideas as to what is the cause and if it is detrimental to our fishes health or contagious, we'd be gratefull.Thanks Kerry

Answer
Hi,
The water is perfect. Cichlids, discus and angel fish especially, can vary their color based off of mood if it's not water quality or tankmates bugging them. Does you tank happen to have a bright light, loud filter, high foot traffic by it, in a room with a tv or anything like that? I'm guessing it's a disturbance from something like that.