I also used to sit pots on top of the flourescent light ballasts, but the seeds only stayed warm during the day(nights are always cool around my area). I now use a padded heater for germinating seeds, tc etc. The heater I use is not the reptile heater, which is very small, produces too much heat and uses 30watts of electricity. I use a heating pad designed for heater sore backs etc. This pad is as large as the bottom of a 10 gallon terrarium and has three heat settings. I usually set the heater on low(probably uses only 15 watts or so, high setting=40watts).