Hey there newbie,
I just recently read somewhere on gardenweb about a peroxide and water mixture. Try a search on here, I believe it was a pretty mild mixture but enough to kill off the gnats. I'd be very careful with it though, too strong and you could burn the roots.

You will probably want to split your infested plants off to a quarantine area regardless of what treatment you do to help prevent further infestation problems. From what I've read and experienced peroxide is very effective for fungal and bacterial problems. I have no experience on how effective it may be with the gnats, nor with other solutions to the problem (I haven't had gnats yet). There is a good chance it could help with the problem but others might have more input.

Over the past few weeks since I first heard of peroxide treatment I've been using it. Here's what I can say if you want to go the peroxide route:

I use a dilution of 1% based on what I read on the EPA fact sheet for peroxide as a pesticide. The kind I buy is readily available in the grocery store and comes at 3% dilution (so I add 2 parts water to one part peroxide to reduce it to 1%).

1) It has, in my experience, been effective as a single application fungicide. I used it on some Narcissus that had developed some fungal problems. I applied the solution to it instead of a regular watering. The four Narcissus bulbs didn't mind it one bit and the fungus died.

*One of the things I read on fungal gnats is that letting the soil dry out helps a lot with eradicating the gnat larvae as they need the moisture. So soaking the soil would seem counter to general advice and I don't know how effective it would be against your pests. As another option, I'd recommend only spraying the top rather than soaking - it'll introduce less moisture and should kill any top side fungus.

2) Hippeastrum bulbs seem to tolerate daily sprays of it with no problems (based on about a week of trial so far). That may be a general indication of how well bulbs in general tolerate it, but I don't know. I've even done a few 1.5% dilutions instead on a particularly unfortunate bulb as an experiment to control mites. I apply the spray once daily to exposed bulb areas and all green growth (or more often as spot treatment if I see a mite) and only water it via peroxide solution soak. It's doing pretty well considering, but I have no real feedback on success other than I now rarely see a mite every two days (down from 3-4 a day when I first caught the problem). All my other bulbs that were in the same general area as this infected bulb are being sprayed daily with no ill effects.

*I wouldn't recommend this for your problem as I imagine the extra moisture of spraying would only encourage the gnats. If you do want to try it, I'd recommend doing it at a slower rate (every other day) -- I leave my plant lights on 24/7 so every day application is more like every day and a half at 16 hours of light to a day.

Hope this helps some and hope someone with experience will chime in :)

Liltika

I've heard the mosquito dunks that you put in pond will kill gnats and the larvie. I just don't know how much you sposed to use, other than break a small piece desolve in water over night use that water to water plants. You can use sticky yellow stake for adults.
http://www.saferbrand.com/store/product.asp?dept%5Fid=60&pf%5Fid=5025&mscssid=7KDMK6UVX1528NU93MEV2SETNVF3DGDD

I recently had an infestation of fungus gnats. I cut back on the water a bit to keep the environment not to their liking, and sprayed with Schultz's 3in1 fungicide/insecticide/miticide... all gnats are dead, no more have shown up... I did combat some of the adults with a flyswatter, just to keep the flying ones at a minimum.