A1555 Glo T5 HO Electronic Ballast for T5 HO Fluorescent Bulbs, Fits 2 by 2-Watt or 39-Watt T5 HO Bulbs Ideal for illuminating aquariums, terrariums, and aviaries Fits 2 by 24-Watt or 39-Watt T5 HO Bulbs For use with 5/8-inch (16 mm) diameter fluorescent bulbs only Energy efficient, low heat output Fully assembled