Alternative to a heater... why? Do you mean an alternative way of heating the water, or an alternative method of keeping them without heating of some kind? They like warmer water.
Some people use a heat mat under the tank, like the kind used for lizards and snakes. IMO I think the "in the water" heater is always the best as they run on a thermostat and stop fluctuations in the water temp, where as the mats are one heat and cannot be controlled. You need to be in a house with a pretty stable temperature to get away with it. Either that or keep the temperature in the room your fish is in very high at all times.....again not really practical.