05-17-2012, 09:56 AM
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In a 4-foot tank like a 55g I would recommend two heaters, absolutely. And either 150w or 200w for each (make them the same).
The benefit of two heaters is that you can have one at each end, preferably one next to the filter return and one next to the filter intake. This means better heat dispersion. And having two heaters mean each is working less hard to heat the water, and will be on for less time doing it. That means longer life from the heaters. Another benefit is that if one should fail, the other will still function and you will likely notice the failed heater before disaster strikes.