Well I know someone will ask about it so---No, I have no idea on how many watts per fish those glow-in-the-dark zebra fish put out.
Anyhow back to MH. I was talking about ballasts and the pulse start vs. probe start. The double ended bulbs, HQI, also have ballasts specifically designed to run them. J.R., aka electic130, was not shot so bad on Dallas that he couldn't provide some info on MH ballasts. Here is a link to his thread on the subject--
MH Ballasts.
It is also handy to see some comparison pictures Joe, JB NY, has a nice series of pics in his thread
MH comparison pictures
Both of the above threads provide a wealth of information on MH lighting and humble my attempts to shed
light on this complex subject.
As with fluorescent, I strongly recommend that your ballasts be remotely located from your hood. If you use a magnetic ballast with MH you probably have no choice as they are much bigger than fluorescent ballasts. The electronic type can be in a hood but they do give off some heat and they are also easy prey for humidity damaging them in that location.
A couple of things if your are making your own hood. Fluorescents can be mounted just above the water surface. Because they get so hot, you need to have some standoff distance for MH. This is usually about 8" from the water surface. This need to have a standoff distance between the bulbs and the water does have some effect on using MH. Try this. Take a flashlight and take about two steps back from the wall and shine the flashlight on it, observing the beam size. Now take another two steps back and observe the increase in the size of the beam. If you measured it you would find that it covers four times the area even though the distance only doubled. Also, since the flashlight didn't increase its brightness the amount of light per unit area is only a forth of what is was. This distance effect is an important consideration in any tank lighting scheme.
If your hood is wood or plastic you want some space between the hood and bulb or you may start a fire. Having a little fireproof insulation between the hood and reflector is not a bad idea.
It is usually a good idea with VHO, but a must with MH, to include some cooling fans into a MH hood. Most places that sell ballasts will have fans or you can buy the type that are used on computers. They are low cost and fairly reliable. For the latter you'll need a 12 volt power supply to run them.
Having a good reflector is also important. To get the most light energy into your tank a quality reflector is worth the extra few bucks. Reflectors come in two distinct varieties depending on what you want to do. If you savor the very directional light you get from MH then a polished aluminum type is best. On the other hand, if you like a more diffuse light, a white mat reflector is what you need. Again, if you check around the Lighting and DIY forums you will find info on reflector materials and design.
Next installment we have VHO vs. MH in a battle to the death.
