Help! What's wrong with my calatheas?

 Continuing the plantita journey, I have acquired few calatheas. 

I am happy to have them and really took my time taking care of them. I followed online tips such as 

-do not use tap water. Use distilled water, rain water or tap water thats left overnight. >> I used filtered water.. 

- if the leaves are curling, they are thirsty. True enough, after watering the leaves u curled

- they need humidity. Good thing I have a humidifier so i run it through them 

- do not put in direct sunlight. Yep I put them indoor beside a window that is not too bright

For the first few weeks, all went well. They looked healthy and started properly closing at night and opened well in the morning. That's really a yey!!

But today... 😥

The leaves of  my rattlesnake curled and turned gray.. And some spots appeared in my shinestar and  rusco. Help!!