That's waaaaay too much power. Either use a regulator as others have said, or a lower wattage bulb. A 40 or 50 watt is more than enough for that size tank. I use a 100w in my 40 gallon breeder on the hot side, and it keeps the temp at a steady 90. A 150 would be too much even for my tank. If the lower wattage bulb doesn't give you the heat you need, try a different fixture. For that tank, zoomed's mini dome with a 50w would work really well I think.
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