Thank you Frank
I plan to do this today, I brought a fish tank scrubbing stick thing, I didn't get around to it before now.
I have observed my discus yesterday and now they are not skimming off anything anymore. I wonder, a few days ago the tank tested at 0.25 Nitrite & 0.25 Ammonia, that is when they were skimming, could this have been the problem?
I tested the water yesterday early morning and the Nitrite & Ammonia had gone again.