hts are the newest variation on fluorescent lights. They are run at slightly higher power levels than standard fluorescent lamps, but are made significantly thinner than standard fluorescent bulbs, which allows for more efficient reflector designs that get more light into the aquarium. Higher quality T-5 systems often match or exceed the output of equivalent power compact fluorescent or VHO lighting fixtures. On the downside, T-5 lighting is the most expensive type fluorescent lighting available. Many times it is much cheaper per watt, especially in the long run with the multiple T-5 bulbs being replaced, to use an equivalent metal halide light setup over a T-5 setup if such high light output is required. All types of fluorescent lighting offer the same efficiency in lumens per watt; it is the shape of the bulb and reflectors that makes their overall outputs different. Metal halide Metal halide lights are generally the highest output lighting c ommercially available. They produce about 90-100 lumens per watt of power. This is roughly the same as fluorescent. The improvement with metal halides is that they concentrate this light output into a very small space, whereas fluorescent lights evenly illuminate the entire aquarium. This is often referred to as point source lighting, and is what causes the rippling visual effect on many advanced aquarium setups. This concentration of light output increases the intensity, allowing metal halide lamps to penetrate light to even the very bottom levels of most aquariums. Metal halides are available in many color temperatures, from 6500 K up to 20,000 K, though bulbs as high as 50,000 K are occasionally found. The downsides of metal halide lighting are the initial cost and the heat produced. Most metal halide fixtures are more expensive than fluorescent systems, but are required for some reef setups. Halide lamps concentrate heat as well as light output. The surface of an operating lamp becomes hot enough to cause second or third degree burns instantly, so this lighting technology must be used with caution. The heat produced can also warm the aquarium to unacceptable levels, possibly necessitating the use of a chiller for certain aqua