It really depends on what you want to keep and the layout or your tank. In the past, a 24 inch tall tank was recommended to have 250 watt
metal halide per two square feet.
HQIs are more efficient than single ended mogul bulbs in lumins and light penetration to the point where many feel that a 175
HQI is close to a 250 single ended halide and 250
HQI is close to 400 single ended. Single ended bulbs are still preferred by some because it seems to have wider dispersion of light whereas the
HQIs seemed to look more focused in light penetration.
More light is generally better, but with an aquarium that is only 12 inches deep from front to back, I can't picture many specimens down low, and if you are running a
DSB or Plenum, you can deduct 3 to 7 inches+ from the total height.
Keep in mind that more watts means more heat, and that would lead to more equipment need, such as a chiller, or evaporation fans. A tall tank does not evaporate heat as well as a wide tank and in many cases, will you even have room lower down in the aquarium for high light demanding specimens? Forget not specimens that demand high light will almost always also demand high rate of water flow.
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