The most effective and safest things that I have used are products containing Bacillus thuringiensis israelensis, or beneficial nematodes.

I usually let the pots dry out quite a bit between watering. that seems to keep the little devils down.

Do beneficial nematodes have a temperature minimum?

Yes they do but it is often different depending on the species of nematode.

Ok thanks very much

I've had problems with plants that I've brought home. First I would get rid of the old infested compost. Replace it with sterile. Secondly, I would wash off all the old compost from your potted plants and disinfect each plant with a solution of 3% hydrogen peroxide. 1 cup of HP to a bucket of clean water, then give each plant a dunk, roots and all. It won't hurt the plants at all but will destroy the gnats, the eggs and anything else in there. Then pot up each plant with sterile compost with new pots. Well, if you want to reuse the old pots, put them in the dishwasher first. The plants will perk up quickly.

If you really want to sterilize the old compost, the best way is to bake it in the oven at 350°F for around 20 minutes. I know it sounds crazy but it works. HP decays quickly into water and oxygen so it's environmentally friendly. Those other chemicals? I'm not so sure. I don't like the stinky & sticky residue from neem oil and it's costly. A bottle of HP is about a dollar at the supermarket and you'll find lots of other uses for it around the house. I use it as a mouth rinse when brushing for pearly white teeth and it clears up a soar throat & mouth sores super fast.

When you say HP you mean hydro peroxide ? Thanks for all your info. I must admit that I've had plants inside before but never suffered an outbreak like this. Neem oil seemed to have absolutely no effect for me.

Neem oil works by derailing the larvae's ability to shed their skins as they grow, so it has no effect on the adult gnats. Since the larvae are living in the potting compost, the only way to reach them is to soak the compost with the neem oil, which would be a bad thing for the plant's roots.

Yes HP is Hydrogen Peroxide. I would definitely dump those pots of infested compost. That stuff belongs outside, not in the house. Go get some sterile compost from the nearest shop.

I agree with Bathgate and use hydrogen peroxide added to water. This works with all my seedlings but indoors especially and was information originally obtained from an article on growing daylilies from seed.

Ok thank you all for your replies