HELP?!?! Need Advice on repotting and gnats

Ok this is my first actual post, been a member of this site little over 2 months now. My grow started out good, but now im running into problems. Started when i used yeast to produce C02, well i guess plants are not the only ones that like yeast and C02 so now i have fungus gnats. Also I am beginning to get brown spots on all of my leaves which may be nute burn, or may be the gnats (not sure). Any way make a long story short, Do you think repotting my babies (roughly 3 weeks old) into better soil (getting fox farm tmorro finally found a hydro store in my city) will help get my plants back on track? And how do i go about doing this? Last and final question... Should i mix a organic pesticide in the new soil to help rid of the lil annoying gnats? and if so What kind?

All and any help is greatly appreciated. ty in advance

Info about grow: Transplanted in 3 gallon pots, closet grow w/ 4 48inch flos (2 HO Blues, 1 Flora Grow which is red, 1 cool white) lights 4 inch away from plants, added blood meal to pots once, foliar feeding at 2 week intervals w/ TerraCycle organic plant food, temperature 75 - 80 degrees, room lined w/ foil (dull side out)

You don't need to add CO2 for that little grow with those little lights.

Keep ventilation and air flow going.

Get rid of your yeast mix. It's smelly and uneccessary.

Hose down your plants with neem spray. Mix it as directed and use a sprayer with a very fine mist. Mist it gently onto the leaves, let dry, mist again 12 hours later. Do this 2x/day until your leaves have a nice healthy sheen to them. This is also an excellent preventative measure against fungus, mites, and other pests.

Many flying pests breed in the soil itself. Yellow sticky traps stuck right into the dirt work like a charm to trap and kill emergent flies, plus adults who are going to the soil to lay eggs. They are cheap and VERY safe.

When you have the problem under control, continue to use the sticky traps to monitor what sort of bugs you've got around.

If you are getting a bunch of spots...that could be spider mites, but not sure...without piks, any help is a crap shoot
peace