Those nats lay larvae in the soil, which will eat the roots before they come up to the top and turn into gnats and repeat the process..
Check out under the treads I have posted, you'll see one on the SUNDEW! Its a carnivious plant that ROCKS at catching bugs. I used to have a gnat problem from MG soil, these sundew had like 20 gnats per leaf stuck to it =)
Cleared my gnat problem =)