03-08-2012, 07:15 AM
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The numeral is the number of 1/8 inches in diameter, so a T8 is 8/8" or 1" in diameter and a T5 is 5/8" diameter.
The vast majority of T5 fixtures, and bulbs, are T5HO, the HO stands for High Output. These are higher in wattage and much brighter than the others. They were designed for use in Reef Tanks (saltwater) so you could get the bright light needed without excessive numbers of bulbs.
T8 and T12 are roughly the same in light intensity, the T12 is pretty much going away with T8 being the standard used in nearly everything.
As for which is best, it depends on the tank. A basic aquarium will do best with a single T8 bulb that spans the length of the aquarium. If you have live plants and you want to do a low-tech setup but have moderate light, a dual T8 or a single T5HO is good. If you want to go high tech (meaning CO2 injection) you would use a dual T5HO.
Too much light (both in intensity and duration) results in lots of algae, and it should also be noted that the these fish greatly prefer, and have the best coloration, under subdued lighting. Even in planted tanks where you want the higher intensity for the plants to grow, you should use floating plants to provide shade and relief for the fish (who should be the primary concern).