Microsorum Trident not doing well

[Bolota]

Hello,
My M. Trident has small dark spots almost everywhere and it is not doing very well. I have in the same tank (low tech) other ferns (Microsorum and Microsorum Petite) as well as Bolbitis and Anubias and all are OK.
Any idea?

thanks

 

[Edvet]

We can do more with pics, overview and plant, and more about your tank.