Discus turning Dark

[Fighter]

My Discus is turning Dark. I know this is common disease which discus face. But there could be many reasons for this but I dont want to start any medication before knowing the disease as discus is very shy and delicate fish. I am totally new to discus keeping earlier i used to keep flowehorn. I brought 6 discus of same size from same guy a week back i.e. 26th. Earlier they were very shy, Within two days i.e sunday evening they started moving round and started eating food. But from yesterday one of the discus started changing black like pitch black compared to its prior colour.
I dont why this happened suddenly with it..Is it because of stress or something else. And from today morning other discus also started showing some shyness or fear.

Other thing to note: Mine is a tank with just one filter and air pump and I changed water on Saturday morning like 30% and other thing there is one of my office colleague a bitch who tapped on tank when i was not around which scared fish. I have attached pics can some one tell me what the problem..And now I m afraid to change water and also clean the poop which i does everyday after they have their food since it will again scare the fish. I have also attached one photo 20180202_221750.jpg to see who it looked earlier.