Hi melosu, sorry for the delay in replying - I usually get prompted when there is a reply but for some reason I didn't this time and haven't been onto the site to check this week. The water I use is all RO. I have noticed some drop shaped brown marks appearing now aswell, this is the first time I have had this. I imagine it's due to the light especially as we are at the peak of summer here in England!! It's not a problem and wipes off easily, I just have to avoid crushing the spirorbid worms at the same time, which can be a little tricky
Got a water change planned over the next few days so I will remove it then. That's if I can safely do the water change with a stressed feather duster worm in the tank - arghhhh. Have posted another thread about that on the sick fish forum!! Do I need to use a new cleaning cloth at every water change? Thanks for your continued help.