I know this thread is a little pointless without pictures but I've noticed that I have tufts of black/grey fibres 'growing' on my background. At first I thought it was dirt deposited by the filters but it doesnt come off easily and is quite fibrous and they dont come off during cleaning. They look like little tufts of weeds. I'm guessing its an algae of some sort as it also grows on the plastic of my internal filters if I don't take them out and clean them periodically but was wondering what type.