It's been more than 30 years since I used gravel, i use fine sand often with a thin layer of soil beneath the sand, i have thick growths of plants and have no problems. black sand will make the fishes colors brighter sand stand out better. Most fish live on sand or mud in nature, many fish like to pump sand through their gills and many feed this way. Gravel only serves to hold uneaten food and dirt between the grains. Dirt and uneaten food sit on top of sand. MTS snails are great in sand, they keep the tank clean and only appear after lights out. Sand is the best way to go for a natural looking and healthy aquarium.