I agree with HUKIT about the 5wpg guideline for average tanks... of course, that falls through a little in bigger tanks. A 150-200w is perfect for a 29. I use a 200w in 29-40, then bump it up to 250-300w for 55g on up to 100+... I have a 300w in my 150, though, and it heats just fine (and not on all the time).
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