I agree with Jon. The watts/gallon rule is over rated in most cases. Different kinds of lights put out different light intensities. How many bulbs you can run also depends on the footprint of your tank. 4 bulbs might be a tight squeeze over a standard 20 but 3 is definitely doable. Also keep in mind that using an IceCap ballast will overdrive the T5 bulbs by about 30-50%, I believe, without really shortening the bulb life.