Zygopetalum with weird black margins on leaves and black spots

[Syngonia]

Hi everyone :-)
I am a newbie here (and not very good with english) and my zygopetalum brought me here! Recently, I bought this one in my local hobby market (I know that’s the bigfesr mistake but I sometime buy orchids there when I am in a “playing flower doctor” mood and give them a chance, heal them and send to the world as a nice personal gift) and I literally don’t know anything about this kind of orchids (Phalaenopsis lover here) but I liked this one and it turns out that something is wrong with it. When I bought it, it already had some brown-ish leaves but I gave it a shot ‘cause you know, Samaritan saving ill orchids and stuff... but I only have experience with Phalaenopsis diseases and how should it look like and how not. I would be very grateful if you could please help me and put me in the right direction.

The leaves have brown/black “margins” and sometimes a brown/black spot here and there and in the middle of the leaf. Also the pseudobulb does not seem very healthy to me and there are also brown spots in the flower stem and the flowers are turning yellow and falling down...

I asume there already was a problem in the hobby market because they do not give a very good care to the flowers... especially orchids. This one was the healthiest looking of the pour zygos there.... But I am not helping her majesty either. A problem I know about is that our water is too hard. Any ideas how to make it better for this plant? I let the water stand on the window for at least a day, but I don’t know if it’s enough. My other orchids are okay with than but this one is

And the second problem is that I don’t know which spot is the best for the plant. High/medium/low light...?

But my main concern is - could it be contagious? I fear of the mosaic virus because I have many other very sensitive plants (and many phalaenopsis orchids I saved from death) and don’t want to lose them. :/

Thank you so much for you help. ❤️

Have a very nice day!

Sending some orchid love
Daniela