But they live in the dirt. The ones they lay eggs are already in the dirt.

If you can find some Gnatrol will get rid of them and not harm the plants. They sell it here http://www.bghydro.com/BGH/items.asp?Cc=PC&Bc=

As well as pure Neem oil. They will work but the think you need to know is you need to treat the plants every 3 day's for about 2 weeks to break the life cycle of them. You may kill the ones you see but the ones you don't see yet are just as big of a problem.

That link also have a room fogger that can kill them as well.

That's about all the help I can give you but it's also all the help you need to get rid of them too. As for homemade remedies not all of them work for gnats, most are for other mites or insects that I know of.

Good luck