My discus is dark

Hello guys I am from Germany that explains my English. i got my 10 x4“ fishes since one month for my bare 100gallon tank.
From the day one i had one fish turns always to dark almost black then became normal and it eats good until befor yesterday it is always dark and it not in balanc
And there was always this something on its skin like peeling skin and now other fish starts to turn dark but all eat good but they itchy.
my water parameters are good
Ph:7
Temperature:29
Tds: 200
Amonia nitret :0
Nitrate:30
I do 50% WC every week
can someone help me?
Thanks

Are the Discus the only fish in the tank? If so, I would up your WC to 50% daily and treat with Furan ll according to label directions.

Up your WCs to at least 50% daily untill the problem resolves. Did you have any other fish in this tank before discus or bought some in a meanwhile ....my point is , could this be cross contamination ?
If it's not , then propably a low water quallity is the first one to blame (good bio filtration and waterchanges ).

I Would start with daily large water changes with aged water (the more the better ) and a regular salt 3 to 4 ppm (3-4: grams of salt per liter of water ). I f this does not get them better in a couple of days , I would start with some general antibiotic , such as Nitrofurazone.

P.s. Photo or a video of them would be helpful too .

Good luck .

Thanks guys

Let us know what you did and how you fared,