Aquarium Heaters?

I use a Penn-plax therma-flow heater. It was cheap when I got it and has lasted me about 10 years so far. I don't rememer how may watts it is, but it was intended for a 10 gallon tank. I used it for a 10 gallon tank for a few years, then a 20 gallon tank for a few years, and now a 29 gallon tank and it still manages to keep a suitable temperature.
 
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