Metal halides are essentially a very powerful incandescent light bulb. Touch a 100 watter in your living room lamp and you'll see my point. Now, magnify that to 175 or 250 watts and you begin to see the impact on water temperature. Maintaining water temperature at or as near to 76-77 degrees is problematic enough. Hang that heat-radiating bulb six inches above the water surface and watch your tank temperature climb, Climb, CLIMB...
Fans, chillers and building special hoods to accommodate MHs are but a few of the residuals that using these bulbs can create. If you are not willing to shell out $500.00-$600.00 for a commercial chiller, then using fans mounted at the water surface or in the hood are your only avenues of keeping the water temp within limits. Ever hear of the domino principle? Fans cool the water by evaporation. If you are replenishing a quart or two a day due to ambient evaporation, just wait till you kick that fan on! Doubling or even tripling current evaporation rates can be realized when using cooling fans.
One thing DOES lead to another here, doesn't it? Did I mention special hoods for MH's? Yep. A 12 inch MINIMUM distance from the MH bulb to the water surface is the rule. This allows some air movement between the bulb and the water, lessening the heat impact. Still gotta use those fans, though!
So if VHOs aren't conversationally correct for your project, and metal halides aren't practical, what other avenues do you have? Power Compact (PC) fluorescents are the answer
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