problem with metal halide

[yerfdoggydog]

i do have a question of concern... will i be able to put in any corals with the 10k halides? just wondering. please let me know also. thanks.

10k and only 100watt, I would stay away from the hard corals since 100 watts in a 75 gallon aquarium is little over 1 watt per gallon and many corals and clams require 4-5 watts per gallon. Also wattage should be considered with aquarium depth. If you have a tall corner tank then even higher wattage should be considered. Fish will work fine with the lighting, however, you will most likely loose color in leathers and other brightly colored corals. I currently have 2x96 Power compact, 2x55 power compact and 1 250w Metal Halide 10k for my 92Gallon Corner. I wanted to keep some clams and had to keep adding light.

[Samper]

The watts per gallon rule really doesn't apply but the depth is what should always be considered.

[Poseidon]

Whoever came up with Watts per Gallon should be SHOT! How useless!

[Ninong]

Wattage is a measurement of power consumption and that's about it. It is not a meaningful representation of the efficiency of a particular lamp. The only thing that matters is the PAR (photosynthetically active radiation) incident on the coral's surface. That can be measured with an Apogee PAR meter (about $400) or approximated with a simple luxmeter (about $100) or you can simply accept the test results published by reliable sources, such as Dr. Sanjay Joshi, for a relative comparison of the various lamps and ballasts on the market.

If one were to have a fixture with a single 175w Venture 5500K metal halide lamp over their tank they would be getting only one-third the PAR that they could get if they changed the lamp to a 175w Ushio 10,000K. Even though they are both 175 watts, the Ushio puts out three times as much PAR.

Furthermore, if that single 175w Ushio lamp is located 6" above the surface of a 20-gal tank it delivers exactly the same PAR to a coral located 8" below the surface as it would to a coral located the same distance below the surface in a 60-gal tank. Obviously the watts per gallon would be three times as much for the 20-gal tank but the amount of light incident on the coral's surface would be identical.

Those are only a couple of examples that highlight the shortcomings of the archaic watts per gallon rule of thumb. We haven't said anything at all about the huge differences in PAR between say 20,000K lamps vs. 10,000K lamps or the very strong Iwasaki 6500K lamps. I believe the watts per gallon rule dates back to a time before VHO fluorescent lamps and certainly before metal halide lamps. It may have had some significance as a guideline for newbies back in the day when the only lamps available were normal output flourescents. Why various "hobby authors" continue to perpetuate this nonsense is beyond me.