Yellowing gravel, I have only seen this in a tank that I used ammonia rocks in and they turned yellow when the rocks were unable to absorb any more. Wipe your finger across the stuff and tell us if it comes off easily and if it has a strong odor to it. If it does then most likely diatoms and lighitng will help, as the tank agaes it will also help as the film develops on the glass and the diatoms lose their food source.
forum community dedicated to tropical fish owners and enthusiasts. Come join the discussion about species,breeding, health, behavior, aquariums, adopting, care, classifieds, and more! Open to fish, plants and reptiles living in freshwater or saltwater environments.