I agree with most of what was said however... Teeb, I think you are doing too many water changes...allow me to explain. You can export phoshate, debris, cyano without removing so much water (every so often) and consequently removing your nitrofing bacteria --> you could cause ammonia poisoning.
Someone said to add polyfiber, this is recommended by bob fenner to do just that. I would take a turkey bastor and blast any and all cyano off your rocks and sand etc. and an hour or so later throw away the polyfiber and replace it with new. repeat every day until it doesnt come back. This stuff is super cheap and every effective. Good luck!