T5 High Output Bulbs are the newest type of lighting for the U.S. aquarium industry. Widely used in Europe, these bulbs have been recently introduced into the United States aquarium industry. At only 5/8" in diameter (approx half the size), it is a much more compact and energy efficient alternative for VHO bulbs and compact fluorescent/power compact bulbs. They boast the highest lumens per watt output of any fluorescent lamp on the market.