05-03-2012, 11:33 AM
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I agree on a 200w, though I wuldn't quibble over a good 150w in a 40g.
But the main thing is "good." Do not cut corners on heaters, they are the most important piece of equipment, aside from the tank itself. Filters can fail and you will likely see it in time (unless you are away of course), but a failed heater can kill the fish overnight, by overheating or failing to heat at all. I speak from experience.
The better--whcih generally though not always means more expensive--heaters will be better made, and less likely to fail. This is where the higher wattage also helps, as a 200w heater will be more reliable than a 50w. My heaters that failed were 50w. And one failed by staying on overnight and literally cooked the fish in a 30g tank. One learns, sometimes the hard way.
Expense is not the only criteria, as one of my most expensive heaters was a Fluval with the digital temp screen and it failed after a few months. I have had good luck with Eheim Jager heaters. My two Tronic have never failed in 12 or more years.