diatomaceous earth is actually made up of the very tiny shells of diatoms which are a form of aquatic life and the way they function is the shells are very tiny and very sharp so crawlies basically cut themselves to pieces crawling around on it.
...the problem though is the same as it is with sand, it works it's way into your media when you water so you'll need to reapply, with the hydroton or lava-rock, or gravel, as i suggested earlier in the thread, you eliminate the environment the larva need to live and if the larva can't live, no more fungus gnats, it really is that easy.
it's vital to understand the life cycle of each pest you encounter so you'll be able to plan effective strategies.
bozo