Aquarium Gravel not only provides aesthetic appeal to your fish tank and a substrate for live plants and aqua scaping, but it also has a number of other benefits. It aids in biological filtration, providing a substrate or "home" for the beneficial bacterial colonies that help keep your fish healthy. A gravel substrate is ideal for bacterial colonies because of its large surface area. In addition, by providing your fish with gravel you are immitating their natural environment which will reduce stress levels in your fish. Stressed fish will often have compromised immune systems and will there for me more susceptible to disease.