Question
I have several diferent types of house plants.  I have a nat problem.  I am not sure which plant they are in but they are driving me carzy.  I have tried the soapy water and did not work.  Is there a nat killer?  I live in Spokane, WA.  

Answer
gnats(flies) are a sign of keeping the soil too wet. they breed in wet soils. let your plants dry out sufficiently between waterings. water by weight. put a saucer under the pot. water and let the plant sit in the excess water for 5-10 minutes.  after that time pour off any excess water in the saucer.  pick up the pot.  it should feel heavy.  don't water again until it feels considerably lighter in weight. the soil will be light in color/not dark and wet.  outdoors check the plants everyday. indoors check it every 5-10 days.  watering frequency depends on many factors(sun exposure, wind, day temps, etc.).
you could buy a safer soap pesticide(professionally formulated) and apply it as a soil trench. they make yellow sticky plastic cards to put near the plants to attrack the adults. you could sprinkle a teaspoon of turf lime over the soil to sweeten(raise the soil pH) of the soil. acidic soils attrack gnats.
get to a local garden center/nursery and quiz them on a solution to your problem.  buy your products from them and they should be more than happy to help you.
good luck.  please rate my answer on the allexperts.com site