fruit flies?

!! Everything's been going fine with my first grow so far, except now there are tiny black gnats in my indoor grow cabinet! They look just like fruit flies. I checked the grow faq pests section, but none of the listed pests seem to be what I'm describing. So far I have seen no apparent damage to my girls. A couple fan leaves that were really covered by dense foliage and shaded from the light have yellowed, but otherwise they look healthy. Does anyone know
1. what they are
2. how harmful they are
3. how to control them.

Answers or advice on any of these points would be much appreciated! By the way my soil was store bought, so I presume it was sterilized.

Hey cyborg, are they mostly around the soil and flying around the room? I might say they are fungus gnats. What soil are you using? Miracle Grow is terrible about that imo, Ive had those and they did cause much of a problem but i hear they can as regards to roots. The controling them....diacostamus? earth and the yellow sticky traps. And keep the room clean. hope this helps

ah, interesting. Yes they seem mostly to be flying around the soil and down beneath the foliage. I'm not using MG soil, but I am using some cheapo stuff. Good info. I see the grow faq also says a layer of sand on top may help. Maybe I'll give that a shot. Thanks for the help!
+rep

Dude, this Miracle Grow Organic Choice seemed like a good idea at first, but now that these Gnats/flies............ how do I get rid of them?

This really works...Get a coffee mug, fill it half full of apple cider vinegar and a tablespoon of dish soap, mix it together and set it down between your plants. They drown in the shit, once a day break up the top inch of your soil..in a few days they'll be gone for good.

they are little black gnats right? well i will tell you every grow i have worked through these little bastards always accompanied the grow they never seem to casue to much troube except for one harvest where i had alot of them and mold becan to form from their droppings on leaves i used similar methods as headie described soil churning is important after treatments to kill any larva or eggs these flies have layed also check underr leafs they lay small egs there aswell.