You have many options when choosing fluorescent aquarium lighting. Fluorescents come in numerous Kelvin ratings and wattages. There are three main types of fluorescent aquarium lighting: Normal Output (NO), Very High Output (VHO), and T5. Very rarely will you see a High Output (HO) bulb commercially available (except for T-5's).

Fluorescent bulbs for aquarium use are usually labeled as T12, T8, or T5. This denotes the diameter of the bulb and end caps.

T12 = 1.5" diameter
T8 = 1" diameter
T5 = 5/8" or .625" diameter

Normal Output (NO) -
Normal Output bulbs are standard wattage bulbs that usually come with many stock aquarium hoods. Normal Output bulbs come in varying sizes, including but not limited to, 18", 24", 36", 48", and 72". Refer to the chart below for corresponding wattages.

Very High Output (VHO) -
Very High Output bulbs allow you to fit fewer bulbs over your tank and provide considerably more lumens as compared to NO bulbs. All VHO bulbs are T12's, or 1.5" in diameter. VHO's, like all linear fluorescents, distribute the light evenly across the water surface, unlike Metal Halides which is a single point of light. Another advantage to using VHO lighting is that they generate less heat as compared to Metal Halides. VHO bulbs generally will need to be replaced about every six months, or as according to the manufacturer's recommendation. VHO bulbs require a 1500ma VHO ballast to operate.

T5 -
When purchasing T5's ensure that you are getting High Output T5's (T5HO). All T5 bulbs must be run on an electronic ballast. T5's have a higher lumen output than a VHO, generate less heat, and are more energy efficient. It has also been reported that T5's last considerably longer, between 2-3 years, with less degrade in lumen output. At only 5/8" in diameter, more bulbs can fit under your tank's canopy.