When changing the gravel in an established aquarium, it is important to do all you can to preserve the biological balance of your aquarium. We advise that you reduce the amount of food you feed your fish for a couple of days before and after you change the gravel, re-use as much of the existing water in the tank as possible and use a biological filter aid to help re-stablish the biological balance in your aquarium. Remember to treat any new tap water you use to top up your aquarium with a tap water conditioner.