No, I think I may have misrepresented myself. I siphon as much 'water' as I can from the 'bottom tank' meaning the fuge/sump. I don't really disturb the sand bed in either the fuge or the display, with the exception of some light vacuming. Very light and I don't go poking into areas where I can't see what I am doing.

I do not believe that disturbing the bed is the issue, unless it's something else disturbing it behind my back. I have a dragon goby that keeps 75% of my bed sparkling white. He leaves the other 25% to nature and it has a light brown covering of diatoms. I tend not to disturb that.

I'll go and get my water tested now. Hope AD isn't busy.