An actinic blue T8. Actinic light sits at the short-wavelength end and is used on marine and reef tanks, where it drives coral pigment and makes fluorescence show. On a freshwater tank it mostly just looks blue. Worth knowing before you buy: actinic on its own is not enough light for most corals — it is normally run alongside a daylight tube, not instead of one.
Sizes
- 33cm — 10w
- 45cm — 15w
- 60cm — 18w
- 90cm — 30w
- 120cm — 36w
Match the wattage, not only the length. The tube has to suit the ballast or transformer already in the fitting, and length alone will not tell you that — read the wattage printed on the tube you are taking out.
Replace them about once a year. Fluorescent output and colour shift long before a tube actually fails. One that still lights can be doing well under half the work it did when new, and plants notice before you do.
Run the light on a timer. Six to eight hours a day is a sensible starting point, and consistency matters more than duration.