Or, you
thought they were gone.
Yup.
@jochhe1998 One thing I forgot to mention is a physical barrier. That's game over for them, you done have to worry about them entering or leaving the medium at all.
Go search for an informational video/site on fungus gnats, get their average or minimum size in mm, look for a material (synthetic material) that is the right micron (mesh size) to be just enough smaller than the gnats.
When you transplant (eg: from a solo cup to a 1gal, whatever - I don't know what you've got going on) you line the pot with a cut piece of the material/fabric; enough so that when you fill it with medium and put your transplant in and back fill it, that you can bring the "extra" material up and over the top/surface of the medium and up tight to the stalk of the plant. Tie/wrap it around securely. --Yes, you'll need to tie it with a material that will allow stretch as the stalk grows. I can't remember the name of the stuff I used. But if you want, I'll find it. Eg: of fabric would be "row cover", or even some kind of mosquito netting (provided the mesh is small enough).
Everything I've suggested is from personal experience, and all of them work. It's not complicated or difficult just because I gave lengthy "directions". It's easy and pretty cheap.
Note: you said you were using synthetic nutrients in coco. That's why I'm not suggesting various beneficial bacteria/microbes, and other critters, or "health soil biology". Especially when you've been, and want to continue it seems, using h2o2...
You've got tons of suggestions and angles to choose from and now it seems things are getting off the actual topic.
Anyway, hope you can wipe'em out. They're just extremely annoying.