The soil gnats in your houseplants are feeding on a fungus that is growing in the soil. The fungus is feeding on rotting plant material in the soil. The rotting material in the soil is, no doubt, dead rotting roots. The reason you have dead rotting roots in the soil of your office plants is because (like most folks) you’re over estimating the amount of water your plants need. Since you mention that they are ‘office plants’ I’ll assume that there are saucers under them to keep the water from ruining the carpet and floors. It is also a good bet that they get extra water on friday to ‘get them through the weekend’. The plants would rather there were no saucers so excess water can drain away. You should never leave any of your office plants standing in a saucer of water for more than a few minutes. There’s no need to give them extra water if you are going to leave them for the weekend or even a week. . . Step one is to stop watering your plants so much. Plants need far less water than most folks realize. You need to know that plants exchange gasses (breathe) with their root system as well as their leaves. When the soil is totally saturated (standing in a saucer full of water), the plant is drowning much like we would. You will drown about 1/3 to 1/2 of the root system of most houseplants simply by leaving them standing in a saucer of water overnight. Once those roots drown, they are dead forever (and now the fungus has a reason to take hold). A better tactic for watering houseplants is to wait until they are so dry that they are beginning to wilt and then soak them. If water then fills the saucer, wait 10 minutes and then remove any water that remains in the saucer so you won’t drown the plant. A plants roots can get very dry without dying and, when rehydrated, will pop back to life. In other words, you can drown a plant in less than 24 hours but to kill it with dryness will take a couple of weeks to a couple of months depending on the plant. Think of it this way: You could survive a lot longer without a drink of water than you could underwater without any air to breathe. Since you already have the gnats, you’ll need to drench the soil with insecticidal soap (which you can get at Hewitts of course) and repeat after a week or so. That, coupled with your new plan for not overwatering should do the trick. If they persist you’ll need to kill the fungus with a drench of fungicide like Captan. I would hold off on that though to see if your insecticidal soap/new watering habit works. In extreme cases, the plant may need to be repotted after removing all the old soil from the root system. Since this is shocking to the plant, it is a tactic of last resort.