Definitions of Fluorescent Bulbs
STANDARD LAMPS T-8 & T-12 - SCROLL DOWN TO SEE OPTIONS
T-8 Bulbs - These are the THIN or SKINNY bulbs measuring 1" in diameter. They burn brighter than T-12 but are actually less wattage than the T-12 (i.e. 48" T-8 is 32 watts but a 48" T-12 is 40 watts), thus being more energy efficient. These bulbs may not fire in all ballasts but will fire very well in electronic ballasts.
T-12 Bulbs - These are the FAT bulbs measuring 1.5" in diameter. These will fire in every kind of fixture including non-electronic ballasts.
POWER COMPACT LAMPS SCROLL DOWN TO SEE OPTIONS 
PC Bulbs - Actually nothing more than a straight tube bent in half, compact fluorescent lamps deliver a higher concentration of light energy in a smaller space than standard-length tubes do. Their VHO nature gives them the ability to deliver the right amount of lumens in whatever color temperature (K=KELVINS) you choose.
There are four main color temperatures available for power compact bulbs:
6700k daylight white-Plant Bulbs 6700k daylight white bulbs are generally the brightest of all the bulbs. The color of this bulb tends to be more of a yellowish/white bulb. Plants thrive under this color temperature as will corals
10000k daylight white 10000k daylight white bulbs are more of a “crisp” white light with just a hint of yellow. This bulb tends to be not quite as bright as the 6700k bulb, but can be more pleasing to the eye, especially if actinic or blue lighting is not used.
50/50 Bulbs 6700k/10000k combination bulb. The advantage to a bulb like this is you will get the intensity of the 6700k side of the bulb, but also the more desirable (visual look) of the 10000k bulb mixed into the bulb.
Actinic Actinic bulbs tend to be more of a purplish/blue color. The Actinic 03 (420nm) bulbs really help to fluoresce corals more than any other bulb. It will help green, reds, blues, etc. to really jump out of corals and all colorful fish.
Bulbs can be all one color temperature, or they can have a mix of colors to achieve a desired look. These mixed bulbs are generally referred to as 50/50 bulbs. One tube will light up one color, while the other tube lights up the other color.
ADVANCED LAMPS T-5 SCROLL DOWN TO SEE OPTIONS
T5 Bulbs - are 5/8" diameter fluorescent tubes rather than the standard 1-1/2" diameter. This slim profile makes the Aquatic t5 Fluorescent system more efficient than standard fluorescent tubes. T5 fluorescent bulbs also put out more light than standard fluorescent lamps, power compacts and VHOs.