A better remedy is to put 1/4 of a "mosquito dunk" in a full watering can and drench the soil of affected plants. As you noted, the presence of fungus gnats often indicates over-watering. Also important to their control is to empty saucers under plants within several minutes of watering. Mosquito dunks should be available at hardware stores. Yellow sticky traps may snare the adult flyers. If you want to try the potato remedy, instructions are in this article on fungus gnats: https://www.gardeningknowhow.com/plant-problems/pests/insects/fungus-gnats.htm
Answered on April 7, 2019
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