Just to add about Fungus Gnats and that is Gnat Off. You add it to your food/water at 1ml per ltr and it seals the lava's mouths shut so they can not munch on your roots and it does no harm to your plants at all. It doesn't smell to nice but it does help big time.
I found it doesn't get rid of all of them but it does cut them back big time and like sysadmin has said if you also use yellow sticky traps that will get rid of the adults that make it past the Gnat Off.
Best of luck with your grow mate.