I saw some cheap heating cables advertised in Ward's Biology Catalog
(1-800-962-2660) - for those of you who are interested. The 1992 price for
them was $25.15. The catalog description says " Soil Heating Cable.
Provides convenient, dependable bottom heat . Ideal for trays, flats and
hotbeds. Thoroughly insulated against water and thermostatically controlled
to turn on whenever soil temperature drops below 74 degrees F. Thermostat
can be turned off when bottom heat is not desired. Has 1.5 meter (5 ft.)
cold lead, and 3.7 m (12 ft.) cable. Serves 0.28 square meters (3 sq. ft.)
bed area. 120 V, AC, 42 watts." I'm not sure if this is useful for
aquarium purposes, but I thought I'd pass on the info.