Fluval Marine Glo Bulb, 39W. More light produced per bulb surface area versus other conventional fluorescents. Longer life versus other comparable high output fluorescents. Maintains superior light output over the entire bulb life versus other comparable fluorescents. Strong actinic blue spectral peak. Strongly recommended for photosynthetic coral, invertabrates and other marine life. Visble dominant blue spectrum highlights blue coloration in fish, corals and other invertabrates.