Fungus gnats. I had them pop up a few weeks ago when I was overwatering. I adjusted my watering routine and installed blue sticky cards at the rim of each pot. Now the only gnats I see are the ones stuck to the cards. I've heard people complain about FFof having gnats, but those things are everywhere. If conditions are right they will appear no matter what kind of soil you use. Earthworm castings are indeed poop! The worm's gut is filled with bacteria that break down organics in the soil. The poop is loaded with bio-available stuff. Assuming I hinted strongly enough, I may be getting a worm-casting factory for Christmas! If not, I'm going to build one.