Discus Going Black

Hi this is my first time on this forum and it looks very good. I have had two discus 1 red terquoise and 1 blue moonlight for about 3 - 4 months now and they have just turned black and started to float on top of the tank and are moving VERY slow. I have re tested the water and the PH is 6.8, GH 8 drops and temp is at 28degrees. I changed a 3rd of the tank to be sure the water is okay. I have recently introduced another small cobolt discus which looks te be okay. Does anyone has any suggestions as I don't want to loose them.

First of all Lucas, what size tank are they in?
How often do you do water changes? How much?
What are you feeding them?
And adding another can cause stress, but usually when the fish moves to the top of the tank, it is due to Ph fluxuations, going up and down, or ammonia toxicity.
If your Ph is 6.8 now, could something of happened to make it crash? And ammonia? Have you tested your water for this?