Some dish soaps can alter the PH of the soil. Repotting can always help get rid of gnats. If gnats continue to be a problem for potted plants, look for a product with BTi to put in the water you give your plant (when thirsty.) Agree with above advice about ZZ not enjoying being consistently moist. The roots make tubers that store water, and can rot if kept too moist, or if they get too crowded in the pot.