If you foliar feed your plants then the high nitrogen is not a problem, but if you drench the soil with a rich product then you're asking for larvae. Gnats don't grow on leaves. I've been using Fish Emulsion for years . If you dilute it and apply it correctly then there's no issues. Keep your soil dryer when growing indoors and you won't have a problem with gnats. I may only water my houseplants once every 2/3 weeks through the winter and once a week in the summer months. I see alot of photos of solo cups where the top of the soil looking "muddy", that can be a breeding ground for pests.....only my opinion...