Fungus gnats
Use yellow sticky traps near the soil level to catch the adults, and keep watering with mosquito bits or whichever Bt-i, every time you see an adult fly. Apparently it is most effective against one stage of the larvae, which occurs iirc within 3-4 days of egg laying. Applying Bt-i in those days after seeing the adult will give you maximum control. I soaked mosquito dunks to use on mine. But it's a fact they kept coming back periodically, so be prepared to use a lot of the stuff if it's a chronic problem.
I happened on this design for an electric powered hotbed yesterday. Not sure what "heating cables" are, but the design is interesting for seed starting in a cold garage or greenhouse.
I wouldn't mind wife-ing myself out into the greenhouse if I had one of these.
http://extension.psu.edu/business/st...ant-production