All of the high output fluorescent bulbs in the chart below have color temperatures near daylight or higher and use rare earth phosphors designed to keep their high light output well into the bulb's rated life.

The 13 watt bulbs have a GX23 base (two pins) and T4 tubes.  The 18, 24, 36 and 55 watt bulbs have a 2G11 base (four pins in a line) and T5 tubes.  The 54W bulbs use G5 (mini bi-pin) connections at both ends of a single T5 tube. The 96 watt bulbs have a GY10q base (four pins in a square) and approximately T6 tubes.

Also, see the end of this page for information on the Color Rendering Index (CRI) and the lumen output of these bulbs, for explanations of the asterisks in the chart, and for general info about fluorescents.

Bulb Chart

*  The Length column provides the approximate length of a twin-tube compact bulb from the end of the glass tubes to the tip of the base pins, and of a linear bulb the entire length including the pins at both ends.  Note that the length of the 55w bulbs varies. For the combined length of a 13w bulb with its GX23 socket, add .5 inches.  For the combined length of the 18-24w with a 2G11 socket, add .2 inches. For the combined length of a 36-55w bulb with our moisture-resistant endcap add 1.25 inches. The moisture-resistant sockets for 54W bulbs add .75" to each end.  For the combined length of a 96w bulb with our moisture-resistant endcap, add .5 inches.

**  Bulbs with a Color Temperature of 5500K-6700K or a 2-6-10 or 3-6-10 designation are appropriate for freshwater planted aquariums.  There is no practical difference regarding plant growth, but there is an appearance difference.  Bulbs around 5500K have a warmer daylight appearance similar to early morning light.  Bulbs around 6700K have an appearance more like daylight in the middle of the afternoon under a clear blue sky.  The 2-6-10 and 3-6-10 are daylight bulbs that accentuate the blues and the reds in your tank.

For marine aquariums, you'll want to mix 10000K bulbs with Blue bulbs in a ratio of 1:1 or 2:1 or use the Combo bulbs which have one tube that is 10000K and the other tube dark blue actinic. 

For Blue actinic bulbs we provide a description rather than a color temperature so you'll better understand what appearance to expect.  Those bulbs called Bright/Medium Blue put out most of their light between 400nm and 500nm.  Those called Dark/Deep Blue put out most of their light between 380nm and 450nm.  Both have their primary spectral peak at around 420-430nm.  Actinism is a property of radiation which, in the visible spectrum, has wavelengths between 380nm and 500nm. 

General bulb info:

The  Rated Life of a bulb, as supplied by the manufacturer, is an average of how long the bulb will light.   About 2% light for 180% of the rated life, about 2% light for 20% of the rated life, though by far the vast majority fall close to the manufacturers' averages. The rated life of most tri-phosphor compact fluorescent bulbs is 6000 to 12000 hours and for most high output T5 linear bulbs about 20,000 hours. These ratings are for ideal conditions -- 12 hours per start, ambient temperature around 70 degrees F, never moved or bumped. etc. For an aquarium application, the effective life is usually considerably less than the rated life.  Conditions in your tank are the best guide to when to change bulbs.  There is no need to replace them according to some pre-conceived schedule.

The Color Rendering Index (CRI) for all 5500K bulbs is 91-92.  That's "excellent" in CRI talk. All other bulbs in the 6700K to 10000K range and the 2-6-10 and 3-6-10 bulbs have a CRI in the mid to low 80's which is "good."  Many Cool White fluorescent bulbs (4100K - 4300K) have a CRI in the low 70's.  That's considered "fair."  CRI's below 70 are considered "poor."   Note that CRI is irrelevant for marine setups when the aim is to simulate the appearance under many feet of water rather than to simulate the appearance of colors under full spectrum light.  Blue actinic bulbs used on marine setups usually have a CRI in the 20's or 30's.

The difference between CRI 92 and CRI 84, for instance, doesn't mean that all colors are rendered with 8% less accuracy with CRI 84.   It only means that certain colors that depend on wavelengths that the CRI 84 bulb is low in or doesn't provide in the right ratios will be rendered somewhat less accurately. For instance, orange colors are a common problem area. It is likely that the CRI 84 bulb will supply all the wavelengths necessary to render all colors you are interested in very well.  That's why a CRI in the 80's is considered "good."

A lumen is essentially a measurement of brightness to the human eye and is therefore very heavily weighted to the middle wavelengths of light that the human eye responds to most readily.  As such, this measurement is not very helpful for aquarium applications since the middle wavelengths are the least important to aquarium inhabitants.  To focus on lumens can be very misleading.  For instance, the 55W 5500K bulb in the chart above has 4200 lumens while the 55W 6700K bulb has 4800 lumens.  Yet these bulbs have the same total light output.  The 6700K bulb simply has a little more of its output in the middle wavelengths.