Thanks Samir, i've been keeping discus for around 12 mths and only recently have had the same problem.

I've treated it for anchor gill (twice) and ripped out all my planting in case it was eggs within the roots etc.

From your post it looks like it was my sand base (about 3 inches deep) - i took that out about 1 week ago and the problem seems to have gone.

It's just nice to know the cause of the problem