Adding MH

[readingbabelfish]

I am thinking of adding a 175w metal halide bulb powered by an electronic icecap ballast to my existing vho canopy. I've got a few questions though. The vho bulbs are two actinic blue and one white bulb, each 110 watts powered by an icecap 430 vho ballast. In order to install the metal halide bulb, I would most likely have to remove the middle vho bulb, which is the white one. If I do remove the white vho, and put the mh bulb there, will that be enough white light to offset the two actinic vho bulbs? Also, the metal halide is a single end mogul base bulb, can this be run horizontally? Does it need a shield below it between it and the water? The canopy already has vents and is openbacked. Please let me know what you all think, thank you.

[gregony]

175W metal halides are being obsoleted. If you're looking at a cheap used one that's OK. But please, for global warming's sake, don't buy a new one, they have no advantage over T5HO whatsoever. A 175W 6500 Ushio produces 11000lm. A 10K Ushio produces 7500lm. Two 54W 6500 GE starcoat bulbs produce 4800lm each and are rated to last over 20000 hours. You can buy a two lamp fixture for around $100. The bulbs are about $15 each. Read all the stuff on T5HO before you add a 175W MH.
I assume you have a 48" tank with the 110W VHO's. One 175 W will only illuminate half the tank. But it should be a white if you've got two actinics already. A lot of people run the 14K and 20K bulbs too. I think the 14K look the best, but only you know what you like. It's not so much about balancing the actinics, most people like a 50/50 mix, but the 14k and 20k bulbs contain so much blue you don't really need to match actinics with the white if you run those high color temp bulbs.
Most moguls will run horizontally and vertically. The bulb manufacturer will specify which orientation it is to be run in, so make sure you check that before you buy. I've seen metal halides run with or without shields. I think you should have a shield, if you have my luck, you will want the shield. Install a fan, you can always choose not to run it if you don't need to, but with the VHO's and the MH you will probably need to.