so you have no recommendations for wattage on a fluorescent tube for
plants?

you'll be limited on wattage to 18 or 20 watts, limited by a 18" bulb...
you'll be limited on spectrum from 5000k to 6800k maybe you'll find one that
is 10000k, but probably not...

Be that as it may, you'll probably like anything in the 5000k to 6000k
range, as the higher bulbs can be yellowish to green, and the lower are more
white to blue.... I tend to want to stay on the lower side, the brighter or
yellower I go, I have algae probs, and CO2 has to be adjusted.....

Also, if you are changing bulbs, you'll run the risk of getting some hair
algae if your light has already lost all it's potency, and you add in a
brand new bulb.... sometimes decreasing your amount of light per day can
subside that....