Hi, im helping someone who recived a neglected tank a few mounths ago

its a 350 l tank with one discus 2 asian bees, 2 plecos and 2 puntius.

the discus started to develop greys black patches on his skin mainy around his mouth

and a bit on his back. its a bit hard to see but heres a pic

What do you think this is? the water quality wasent so great lately due to lack of WC but iil get on that now, did a 40% yesterday and will do 25% every week now. , nitrates was 40 befor the last wc

In addition i would appreciate any tips on keeping discus, i know ph and hardness are a deal with them. thank you!