# What is the difference in metal halides ?



## spinxarelli (Feb 24, 2006)

I am having a tough time trying to understand the difference between HQI metal halides and screw in Mogul applications. Does one work better than the other ? Maybe a list of pros and cons would help me in choosing between the two.


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## AaronT (Apr 26, 2004)

The HQI bulbs are a much smaller profile, which helps to reflect more light into the tank when proper reflectors are used in conjuction with them. Much like a T5 bulb over a T8 or T12.

The other difference is that the Mogul bulbs have a UV quartz jacket and do not need a pane of glass between them and the tank. UV protective glass should always be used between the HQI bulbs and the tank to protect the fauna, flora, and yourself. 

It also makes the fixtures look sleeker when using the low-profile HQI bulbs....not really helping plants..but it ups the cool factor.


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## spinxarelli (Feb 24, 2006)

If I have a mogul style fixture (let's say a pendant) and I want to keep my glass canopy on my aquarium,will that do any harm, perhaps as blocking out beneficial light or will the the halide's bulb penetrate right through glass canopy without any interference. The reason I ask is because I would anticipate some serious evaportion without my glass canopy. But if it is going to strip my plants of some of the light's intensity then it is worth taking off my canopy. Thanks Aaron.


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## AaronT (Apr 26, 2004)

No, keeping the glass on shouldn't block any light, provided you clean it from time to time. The biggest hurdle when using MH bulbs is heat and evaporation like you said. Make sure to hang the fixture at LEAST 12" above the top of the tank to keep it from warming the tank too much.


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## Mjproost (Nov 17, 2005)

> No, keeping the glass on shouldn't block any light, provided you clean it from time to time.


Are you sure about this? I cannot find it right now, but I read an article on this a couple years back. I remember this article stating that regular glass blocked almost 20% of the light, more if its dirty. Logically, I would think that unless it is optically clear glass, it would block at least some of the light.



> The biggest hurdle when using MH bulbs is heat and evaporation like you said. Make sure to hang the fixture at LEAST 12" above the top of the tank to keep it from warming the tank too much.


True heat can be an issue. But, the MH over my 37 gallon open top doesn't increase the water temp much at all. Maybe a degree. But the CF over my 20 gallon in a regular hood raises the temp at least that much if not more. Also, all my tanks seem to evaporate about the same. It seems to have more to do with atmospheric humdity and the temp of the water than anything else for me. But, I do not really have any tanks with tight fitting tops. All have at least have the back open. I topoff twice a week.

For me the biggest issue with MH is even light coverage and shading because it is such a small point source. Even with a 250 watt MH I find it difficult to grow foreground plants like glosso unless it is right in the middle of the tank directly under the light and unshaded. Also, my plants always seem leggier under MH.

That being said I love my MH lit tank, the sparkle, waves on the ceiling and being able to always look down on the plants is definitely worth it.


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## AaronT (Apr 26, 2004)

Mjproost said:


> Are you sure about this? I cannot find it right now, but I read an article on this a couple years back. I remember this article stating that regular glass blocked almost 20% of the light, more if its dirty. Logically, I would think that unless it is optically clear glass, it would block at least some of the light.


Well, it likely does block a little bit of light. I doubt it is enough to warrant keeping the glass lid off, if one really wanted to keep it on. I agree it's very important to keep it clean.

I had 2x175 watt MH bulbs on my 75 gallon tank for a few months and heat was definitely an issue. Part of the issue was that the fixture could only hang about 8" above the surface of the water. I recommend making it higher than that. As for light spread issues I recommend having one bulb per 2'x2' space of aquarium. That should be enough to grow anything.


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## almond (Mar 5, 2006)

its better to leave the cover on....or you're gonna have to top the tank up like every 2-3 days.


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