Weeeellllll, There could be both, mites and gnats. The yellowing sounds premature, so it could be mites causing that. Look on the bottom of the leaves.

How to prevent? You have to use a powerful insecticide at any opening to the grow area. Air intakes need to have a good dose sprayed in when the fans are off. All corners of the room after washing the entire room with a chlorine/water mix.

Then, during Veg, at a minimum of once a week spray with Neem oil, about an ounce per quart, shaken, water should have set and not have chlorine. Spray the leaves. Once you go into flower, you want to not give too much after the first 3 weeks. You can also put about a teaspoon to tablespoon in a watering and give one dose in late vegetation directly into the soil, the plant will uptake enough to keep the mites away. You may also read the link I have on mites.

Gnats, are a real PIA but not something that will damage the plant. I found outdoor soil brings them in. Regardless, I keep a sticky fly tape in the rooms to get the stuff that flys.
WashougalWonder Reviewed by WashougalWonder on . mass exodus of mites from my soil after a flush... hey everybody! so...i flushed for the first time this morning, and upon returning plants back to grow area (bedroom closet operation) and getting them re-situated, i saw HUNDREDS of little light-brown mite looking things crawling all over the soil and up the rims of the pots. never have seen them until this. spider mites? or something else? there were seriously tons of them...if they are spider mites and decide to cruise up into the foliage i think i'm fucked. anyone have some insight on my Rating: 5