Fluorescent bulbs are used in aquariums to promote plant growth and to bring out the full colors of coral reefs and tropical fish and invertebrates. Fluorescent bulbs are generally much more efficient than incandescent bulbs because they produce more light and less heat. The brightness of a light bulb can be measured in light output, or lumens. Depending on the type of lamp and efficiency, flourescent bulbs are rated initially between about 45 and 100 lumens per watt.
Fluorescent tubes or bulbs produce a certain amount of light according to their wattage and their efficiency. Bulbs from different manufacturers rated at 40 watts will produce different amounts of light. If a bulb is more efficient, it will turn more of the electric power (watts) into light. A lumen is a measure of light output. For typical aquarium use, a "grow light" or "plant light" designation will give you the type of light needed for plants and to bring out the full colors of your specimens. For a typical 40 watt fluorescent bulb, the initial lumen output will be between 1600 and 4000 lumens---check with the manufacturer for the specific bulb you want to use.
Bulbs are rated in initial lumens and design lumens. Initial lumens is the brightest the bulb or tube can be, and design lumens is the amount of light they output after being used for a short period. The initial drop in brightness is about 20 percent. Lumens drop rapidly after initial use, then slowly decrease for the rest of the life of the bulb. Check the manufacturer for estimated bulb life. The bulb will produce about 50 percent of the initial lumens at the end of its useful life.
Fluorescent bulbs produce light of different "temperatures" according to the gases they have inside and the coating on the bulb. The coating, made up of phosphors, absorbs most of the light produced in the bulb or tube and re-emits it at a longer wavelength---this is called "fluorescence," and hence the name for this type of light. The temperature of the bulb, measured in Kelvin degrees, tells how close to sunlight the bulb's light is. Sunlight is rated at around 5500 degrees. Typical fluorescent or incandescent lights are rated at a "warmer" or more red/yellow 2500 to 3500 degrees. For aquarium plant health, you want to have a fluorescent bulb that matches sunlight temperature or is even higher. This provides the bluer light wavelengths needed by your plants to allow healthy growth.
The amount of light you need for a tank depends on what your purpose is. If you want to brighten the fluorescence of a reef tank, you can use a lamp with a higher color temperature than daylight---up to 10,000 K, and put as much light in the hood as possible. Natural fluorescent compounds absorb shorter wavelength light (ultraviolets and blues) and convert it to fluourescence at higher wavelengths (blue, green, yellow, orange, and red). That is why "black lights"---ultraviolet lights---produce such vivid fluorescence. They provide the short wavelength light needed for the colors to fluoresce.