Ok, i have had a horrible time trying to keep my gravel vac clean, and i took up the suggestions here of putting vinegar in it...Any way, the black substance, (mold or some other stuff :look partially cleared out (ok not much at all) and i tried alternating vinegar w/ hot water. It seems to help only minutely, so Any suggestions are very helpful as i live in a hygenically strict household (mum is a nurse) so it needs to be fixed. Also, the same applies to the inside of the tank. I always try to avoid soap or other likewise products, but i cant help but think there has to be some kind of general cleanser (?) for tanks..either that or get the rubber gloves they use for SW. Thanks