Gravel gives an aquarium a natural and functional form of biological filtration that will control the bacteria levels. And it will create a good place for good bacteria to grow. If you have or plan to have live plants in your aquarium, putting gravel is a must as it will provide a great place for aquarium plants to root and grow properly.  The gravel will make sure that both your plants and fish have a healthy life. However, not all gravel is safe for the fish. Cheap gravel has dye that can discolor the tank water and even introduce a poison to your aquarium and cause damage to the health of your fish.  The right substrate will help provide a healthy habitat for your fish and provide a natural feel to your fish and help reduce any stress.