10-01-2012, 02:17 PM
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With a good tube, the present fixture might be fine. A "daylight" tube with around 6500K rating. If the additional expense is not an issue, a good tube is the Hagen Life-Flo, or the ZooMed UltraSun; both are 6700K.
Liquid fertilization will obviously be needed, as the Cabomba is fast-growing. I will caution that this still might not be sufficient for this plant, but before buying a new fixture it may be worth trying. A 20g long is not as deep a tank so the light will be more intense. But if this should prove insufficient, a dual T8 fixture will be the next step up [a single T5 HO would work, but I don't think one can get single-tube T5 fixtures].
Another option is incandescant [screw-in bulbs rather than fluorescent tube]. If a fixture with 3 sockets is available, using CFL Daylight 6500K bulbs can work. These come is varying wattages, the higher the wattage the more light with CFL bulbs.
One other factor enters this though, and this is CO2. Stem plants, especially reddish ones, need more light and nutrients including CO2, so even with more light the CO2 may not be sufficient. As every aquarium is unique, it is difficult to be precise.