I agree with fishorama. There are not many fish who PREFER large gravel, and larger gravel tends to collect dirt and debris. I would get a medium grade gravel, a fine grade gravel, or even sand. Both cories and kuhlis love sand, and dirt accumulates on top rather than inside of it.
If you do change your substrate, make sure that you save a bunch of it, and put it in a large filter bag or pantyhose, and hang it in your tank. Your substrate cnsitutes a large portion of your biological filter. You would want to leave it hanging in there for at least 2 weeks, then slowly remove it.