01-24-2011, 02:48 PM
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The maximum watts for a fixture is due primarily to the heat issue and refers to incandescent bulbs (screw-in). Fluorescent come in standard wattages for the tube length (though some make lesser wattage tubes now that are more efficient) so this is not usually an issue with fluorescent.
For incandescent, using the compact fluorescent bulbs (also screw in) works best. They provide increased light intensity for less energy, and as watts is the measure of energy used by a bulb, this allows you better light with probably fewer watts.
The other issue is the type of bulb, meaning the light spectrum. Plants grow best under "daylight" with a Kelvin rating between 6000K and 7000K. GE, Phillips and Sylvania all make CF bulbs in daylight, with a kelvin around 6500K. I have two 10w bulbs over my 20g. If your fixture only has one bulb capacity, I would get a higher wattage, like the 15w, in a daylight bulb. If you do have two bulbs, two 10w daylight will be good.
The other issue is nutrients; plants need food, some 17 nutrients, and all are unlikely to be present in the average aquarium. A good fertilizer solves this problem. Seachem's Flourish Comprehensive Supplement for the Planted Aquarium is one of the best in my experience. Only once a week, or maximum twice, and in a 20g a 1/4 tsp will be sufficient so it lasts a long time.