Marine-Glo Fluorescent Bulb T5 T5 16mm diameter bulb; Recommended for marine life Simulates deep marine light; Stimulates marine growth. Visible dominant blue spectral emissions highlight blue coloration in fish, corals and invertebrates Compact dimensions; Two T5 HO bulbs require only approximately as much space as one T8 bulb T5 bulbs require a unique fixture. If you use a fixture other than a T5 Fixture the bulb will not fit or work.