Plants use a lot of certain red wavelengths, and a lot of certain blue wavelengths.
They use a lot less of the greens and yellows.
A PAR meter, or the PAR test results will tell you which bulbs offer the best PAR value.
People see the yellows and greens better than the reds and blues. A bulb that is bright to our eyes may not offer much energy to the plants.
The K value is a number that represents what our eyes see. It is a blend of wavelengths, but just looking at the K value you cannot tell what the different wavelengths are. In a general way you can say that a low K value (couple of thousand) is more likely to have red-orange-yellow wavelengths and a high K value (10,000) is likely to have mostly blues. An in-between K value (say about 4000-6500) is probably going to have some of each. But is also likely to be more heavily weighted in the yellow to green range so that we will see that as a good, bright light.
'Daylight' bulbs might have a better range.
In all of these, look at a chart on the package (if there is one) and you can see which wavelengths are most prevalent in that bulb.
When I used a lot of fluorescent bulbs I would combine a 'Plant' bulb with a 'daylight' bulb so each tank had 2 or more bulbs.
The plant bulb looked sort of dim, more pink-purple. Sure made some fish look funny! The daylight bulb balanced it out so that to my eye the tank looked natural, the fish and everything in it looked right. When I could not get a plant bulb to fit the fixture I would look for a Daylight, or a Cool, or, last of all, Warm. A combination, IMO was usually best, with the idea that what one bulb lacked, another would supply.
Now that PAR values are becoming more common, use these whenever possible. A 'Plant' bulb, or a bulb sold for planted tanks sure ought to have the PAR value on the package. Other bulbs might have been tested, but you may have to research to find it.
Look here at TPT, I think there was a sticky that included PAR values, measured by members, for a lot of different bulbs. Also look at the manufacturer's site. This info might be listed there.
