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15w on a 26 gallon tank is less than 1 watt per gallon. If possible I would go with 2 bulbs for a total of 30w at least. Also, make sure they are fluorescent bulbs as well. Incandescent bulbs are basically useless to plants.
If you simply stick to the 15w bulb on that tank, you will have a very tough time growing plants
A good range for plants is between 6,500-10,000K. A single 15w bulb on your tank would be enough light for some simple low light plants like java fern and java moss. However, it would not be enough light for dwarf hairgrass.