Peroxide will kill on contact.. But i would dilute peroxide and drenche soil to complete run off they rinse off. And would not recomend it in organic soil has it may kill beneficial bacteria as well. I would use neem oil to do a water drench insread. Let your soil dryout befor trying to ride of this sciaride, fungus gnats prefere moist environment so lowering humidity in the grow out en drying soil in between watering will help reduces. B. T. us use a lot in biological growing facilities as it can target many undesirable pest and causing larvea tu starve and die without killing beneficial bacteria... Neem and bt would be my go to choice combine with a moisture reduction