How old is the bulb? Fluorescent lamps tend to shift color temperature with age, people advise to get new bulbs every 8 months or something like that.
With a 16 gal
tank a 15 watt would put the tank at really low light. I think the upgrade to 2 x 15 watt bulbs would work but you need to make sure you get either full spectrum lights or lights make specifically for plants.
It's hard to tell how much light you would need without knowing what type of plants you have.