Whats wrong with my plants!!!

[ashbaxter]

im not sure if im in the right section but most of my plants appear to be stem plants so maybe im here!

On maybe 50% of my plants the leaves are turning a darker colour, almost look like they are dying or getting some kind of build up on them..what could cause this!?

Hopefully that makes sense, let me know if i need to provide more info!

Thanks!!!!

[Lady Hobbs]

If this is a new set up, could be diatoms showing up on the plants. It happens.

[ashbaxter]

tank has been set up and running for about 4 months now. all the plants have been growing really well....