Using a desklamp isn't a good idea because you need to do one of 2 things, both of which are bad, 1) you have to leave it on most of the time, especially at night and the fish will get no darkness, which they need, 2) you turn it off at night so the fish can have some darkness time, but then the temp drops and fluctuation of temp is just as bad as being cold (if not worse).
What I used to use for my small Betta tanks before I started keeping the fish room at a constant 78-80 degrees, was use a heating bad, the adjustable kind you use for sore backs or to keep your feet warm, hehe. Adjustable because then you can pick which setting is perfect for the tank. I kept mine at the lowest during the day and turned it up to the next setting at night when the temp in the room was much cooler...it worked wonderfully and kept the Bettas nice and cozy throughout the day and night.
Linda