I concur about the fungus gnats! They can kill your seedlings!
I found this article about using H2O2 to kill fungus gnat larvae:
http://forum.grasscity.com/sic...
Interesting, because I do use H2O2 mixed in the Spring water I use to water my seedlings when I start them inside on the window shelf. Sometimes I have a gnat problem and sometimes I don't. Must be the amount of H2O2 that I add to the water before watering everything.