T5 light bulbs are fluorescent tubes that are narrower than the typical 1 ½” diameter fluorescent tube. With the T5 designation referring to the diameter of the bulb, T5 lights are more compact and yet more powerful than the standard tube.
The new T5 technology has resulted in a bulb that is perfect for aquarium use. Because maintaining the health of the fish is of primary concern, the T5 which contains a flattering light is a better choice than a metal halide light. Using metal halides can result in a hot spot, an area where the light shines intensely on one area. Not only can this cause harm to the health of the fish, but it can also actually kill a plant that is directly underneath the light beam.
The strength of T5 lighting for aquariums is quite sufficient for an average-sized aquarium found in a home or office. Try to install an even number of tubes over your tank, with half blue and half white. Most aquarium owners have a timer to turn the lights on and off. If at all possible, install a second timer, one connected to the blue lights and one connected to the white ones. Turn the blue lights on ½ hour before the white one and then leave them on ½ hour longer. This will be very helpful in keeping algae under control.
For an aquarium owner to switch from standard fluorescent to T5 light fixtures can be a big investment. This change should be considered a long-term investment. T5 bulbs will last longer, provide greater lighting, and are much better for the occupants of your aquarium. If you currently are using metal halide, and have a deep tank with coral on the bottom, you can more economically switch to T5 by keeping one or two of the metal halide bulbs which will continue to provide the light necessary for the health of the coral. This makes a great compromise in making the move to T5 light fixtures.