Your seed-starting discs could have had eggs in it. I would have thought the discs would have been sterilized though. You can buy Bt specifically to combat the larvae. It goes by the names Gnatrol and Knock-out Gnat. I bought some years ago and unless you have a greenhouses you basically get a lifetime supply. You just mix a tiny amount with a gallon of water and use it to water your plants with. I had an infestation earlier this year and the number of gnats has decreased significantly.
Another way to get them in the Spring is by hardening off your plants. If you set them outside for a few hours and then bring them back in the house you'll probably bring in some gnats also. For older plants you can add sand or chick grit to the top surface of the soil. If the gnats can't reach the soil they can't lay eggs! I'm going to try that this year.
