This article is from the Aquaria: Plants FAQ, by multiple authors.
Fully-automated systems can be purchased from commercial sources such
as Dupla, though the cost can be a bit much for a beginner. You can
save a great deal of money by buying just the cables and building the
rest of the setup yourself. If you use a small enough wattage cable as
a supplement to your tank's main heater, the temperature controller
can be ignored or replaced with a timer, requiring only a low voltage
transformer! Furthermore, it is possible to make your own cables,
taking the price down almost to that of a "normal" heater.
 
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