12-06-2012, 08:54 AM
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The problem with using a light is, it is not on the entire day, and the night is usually when a house is the coldest. Large daily temperature swings are not good for fish. Using warm water from the stove has the same problem, but even more temperature swings throughout the day (assuming your adding warm water more than once).
Unfortnatly there is no 'free' way to create heat, you'll be paying for it in one form or another (electricity).
I assume this is a Betta, as that's usually the only fish people subject to small bowls? Perhaps find a friend who keeps their house in the 70's willing to watch it for a time until you can get a heater. Ideally someone with a wood stove or fireplace as the area around those is usually several degrees warmer than the rest of the house. When you can, combine it with some other small things you need for the house and you can get one off Amazon for less than $7.50 with free shipping. Free shipping can take up to two weeks though.