James wrote:
200 watt in a 37 gal tank

That's about twice what you need. Fenner recommends about 3 watts per gallon;
100 watts should be sufficient. When you have too large a heater, it comes on
frequently; smaller ones come on and stay on for a while ('cause they heat the
water more slowly). This more frequent cycling and more rapid heating of the
water stresses the fish and leads to thermostat failure with mechanical thermostats.

While we're on the topic of thermostats, I would go with an electronic
thermostat. I'm told that they're more reliable than mechanical and, when they
break, they usually fail in "off" mode. My old Supreme Heetmasters used to
require replacement of the mechanical thermostat every few years; I've had no
trouble from the Tronic brand replacements I bought over 10 years ago.

I'm also a firm believer in dividing the heating load. If you have two heaters
and only one fails, there's a much better chance that the tank will survive
until you notice the problem. With your tank, I would go with two 50 watt heaters.

George Patterson
Those who do not study History are doomed to repeat it. Those who DO
study History are doomed to watch every one else repeat it.