I have a saltwater tank for over a month & a half with sand and it started getting this weird stuff growing ontop of it, looked like brown stringy hair ? Not sure if it was bacteria or what.. so I re-did my whole tank, the lady at the store said i should have used natural gravel instead of sand.. so its been about 2 weeks now & its starting again.. idk what it is or how to get rid of it...please help