The number one mistake made, when starting seeds indoors, is not sterilizing the seed starting mix. Peat based starting mixes are notorious for having fungus gnat eggs in them. They can survive temperature extremes and sit dormant for a very long time in the mixes. Once you add water to the starting mix and expose them to the grow lights and warm temperatures, they hatch. The larva feed on the roots of your seed starts and grow into adult flying gnats. The fungus gnats lay more eggs and the cycle continues, eventually causing the huge problem of seedling loss. The best way to deal with fungus gnats is to stop them from arriving.
You want a sterile starting mix for seed starting. You do not need microbes in your mix for seed starting. Using boiling water to hydrate the seed starting mix will kill the fungus gnat eggs 99% of the time. Most seed starting mixes have little, if any, microbes in the mix. They are typically peat moss and vermiculite based. The boiling water will kill any microbes in the mix. However, they are not needed and beneficial microbes can be added later using worm casting tea, fish emulsion, worm casting and other products, if you are concerned. This video shows you how to use boiling water to correct the number one mistake people make when starting seeds indoors.
Concern for soil life and beneficial microbes can be addressed when your seeds starts are maturing and get potted up. At this point, instead of seed starting mix, we can use potting soil that is full of microbial life, add in compost and other amendments, at that time, as you wish. The bottom line is that indoor seed starting creates the perfect environment for fungus gnat eggs to hatch and for their life cycle to flourish. There is no Nature to naturally address the problem, since they are indoors. Start with a sterile mix, add the 'good stuff' later, that is free of fungus gnat eggs.