Its the larvae of the Fungus Gnat. Ihave them in every grow... its like B4RNS says, a sign of living soil. Iam fighting them with Predators. I am using Nematodes. You can order Predators and just give them into your watr and they spread and eat up all fungus gnats larvaes. As i tod you i hav them in every grow , so i fight them regulary. After a transplant i give nematodes in my waterings and they kill the larvaes. If some survivie icatch them with yellowstickers. You can put fine quartz sand on the layer of your pots. The sand is so sharp for the gnats that they get killed if they try to get through it. Same to diamantous earth. If you put quartz sand as layer, you may water your girls from the bottom. To me it dos not work wth sand or other stuff, because i use airpots so i go with a combination of yellowstickers and predators. In your case its to later for predators( they need a few weeks to grow) so i would go with yellostickers to rduce them if they developed from larvae to flies.
+2 strains
