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You will be fine with a 20 watt. Coverage depends on the reflector, but will easily cover 12"x12". More with a better reflector. The spiral compacts are nice, but perhaps you could get a cheap 6500K T5. It would get better coverage and still be low-wattage.
If you are really worried about 75watt difference, perhaps get rid of the refugium all together. They are overrated. Some of the best reef tanks ive seen don't have a refugium. Some don't even have a sump. HTH
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