The DE is for the Larva, it is sharp and they burst when trying to crawl past, I think you should just relax a bit and keep up what your doing, your likely putting a huge dent in the population, short of using an insecticide I think your doing fine, give what your doing a chance, make life for the knats inhospitable eventually they will be gone. But as long as your using soil there is a good chance things are going to come alive in it.