That might work for a little while, but if the room temperature drops like most usually do over night, then the water is going to cool and just go back down to the highest temp the heater will keep it at. Its the same thing with those little pad-like present heaters.
Its never a good idea to rely on lights as a heat source either. You can't control the amount of heat they put out, so they can over heat your tank very easily and they can only be kept on for about 8-12 hours during the day, so as soon as you turn them off the temp is going to drop down at night when it gets colder and a heat source is often most needed.
How many drops are in 1 ml? The water conditioner I have says 5ml or 1 teaspoon for every 10 gallon, so it's 1 ml for 2 gallons right? but I don't have anything to measure it! I can only let it drip out, so that's why I need to know how many drops, I guess I would have to tell you how big are the drops, but I don't know..