Years ago I found out that the same species of Bacillus Thuringiensis that kills mosquito larva also kills fungus gnat larva. So, I put a mosquito dunk in my watering can and watered all my indoor plants with the water after letting it soak a while. It quickly got rid of my fungus gnat problem and I didn't have to worry about the cats drinking the water. BT isn't harmful to anything other than mosquito and fungus gnat larva.
