Pests ( very small flies that live in soil and hide behind leaves

These are tropical bed bugs. How the he'll did you get them in your grow I don't know?! You got a window? Throw that shit away...its you or them...act now!

This usually happens with new potting soil (or maybe when you use cardboard at starting pots). Fighting with it by hand is likely to be long and tedious. ^^ Instead, place yellow sticky traps at ground level. Also, this type of flies likes moist soil to lay its eggs, so it's advisable to use mulch to prevent them from reaching the moist layer. You should see larvae appearing in run-off. Increasing the watering to force more run-off will help get rid of them more quickly. (As you can see, I'm not really into pesticides).

No pic of the flies? Hard to tell what they are, but they sounds like they could be fungus gnats or white flies. BTi or Nematodes are my go to for them. Should cull the population in a short period of time. You can also use yellow sticky traps to monitor the situation and get photos of the pests. Good Luck!

i haven't tried it myself but i have heard you can also put some stuff on the soil, like sand for example, so they will die underneath. if you do this, please google it before to make sure it will really work. make sure to let the upper soil dry out, so they die anyway. also use yellow sticky boards, so they will fly against them and die on it.

I've only had fungus gnats get thick on me once in my 3x3 but I used a diluted hydrogen peroxide and water solution to spray them when I saw them and did a soil drench with the same solution 1:3 ratio roughly, 1 part peroxide to 3 parts water or 4 parts water.

You can use Nematodes, BT Israelis or Mozzy bits, 8 spray also works pretty well...I wouldn't use neem oil personally as I haven't had much luck with it. They are most likely fungus gnats.