What are the alternatives for using gravel in a fish tank?

Any inert material that doesn't give off chemicals will work. You can go w/ no gravel, very small pebbles, large gravel, rocks, most plastics, quartz, marble, rounded gl, other stone, cork, drift wood, etc., etc., etc. Make sure you wash whatever you use. Pour some vinegar on rocks to make sure it doesn't bubble and if it doesn't bubble you can use it.