What I have VERY successfully used for fungus gnats is something called Mosquito Bits... you soak them in your water for 30 minutes (package will give you directions), strain them out (I put mine in a sieve or strainer to make removing them easier) and then water as normal.... If you've got a bad infestation, you should follow their advice to apply this once a week for 3 weeks... I only had to use it once and the fungus gnats were gone - have never come back. You can get these at any garden center or from Amazon... GOOD LUCK!