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When I choose a heater, I figure on 5 watts per gallon of tank volume for smaller tanks up to 30 G and 3 watts per gallon for the larger ones. So, a 150 watt heater should be fine for a 46 G. That's the size I use in my 45 G tall. I keep my tanks at 76 degrees.
I have a 200w in my 46 bow, and in the mid 70's it keeps up just fine. When I had the tank in the mid 80s for cycling it kept up, but it seemed like it was on a lot. I am going to be setting my tank to 80, and I will run two 200w just to be on the safe side.