Why is MH sooooo much better than VHO?


New member

Reading over many posts on this board, you get the definite impression that MH really is necessary in order to keep the more challenging corals/clams and that those poor fools running VHO, in any amount, might as well throw in their salty towel. (Ok, Ok, I might be going a little far here but you know what I mean :))

Case in point:


I wanna get a maxima for my 30 gallon long reef tank, and I was planning on putting him on the substarte. Is this not possible? I have gotten Daniel Knopp's book yet, but I will soon. am I correct in assuming that they must be place don the rocks? The lighting will be four 36", 95 watt VHO lamps for 12.6 w/g. Two actinic, two daylight. Is this good?

IMO, your lighting might be a little weak for a maxima, especially if you have it on the bottom on the substrate. Perhaps it would be better to place the clam a little closer to the light, up on the rocks. In a tank of this size with MH lighting, I would say it's probably close enough to the light on the substrate.


My question is this: Do you not get roughly the same intensity of light per watt from MH and VHO? Why wouldn't more than 12 watts per gallon be enough for a Maxima?

I am currently running 330 watts of 10,000k VHO over my 48 gal. tank on an Ice Cap. I made the choice because it avoided the issue of excess heat that is often a problem with MH, allowing me to place the lights very close to the surface, and because I really do like the look of the flourescents. I love the beautiful colours of the Tridacna clams and have one Derasa on my substrate that appears to be doing very well. The gold / yellow colour is absolutely incredible and it seems to be growing steadily. I have gorgonian, and finger leather (admittedly not the most light demanding species...) growing like weeds and I really had my heart set on a Maxima. I talked myself out of a ridiculously cheap Ultra grade Crocea but I thought I might be OK with the Maxima. There are at least some Maximas found at lower lights/ higher Kelvin temps. amongst coral reefs in the wild aren't there? I do not want to come across as sounding like I will only listen to what I want to hear, but I do want to understand what I and others are being told.

Owen deVries

Note: I have nothing but respect for HFF and his contribution to both this board and the hobby as a whole (I can't count the number of times the sight of his tank has had me shoulder deep in salt water trying to improve my own tank...) and don't want it to appear that he was being singled out in my example, it was simply the first instance that I came across. :D
Their are quite a few reasons why MH is better, but all of them lead to: more light intensity concentrated in a smaller amount of space. Out of PC/VHO/NO/MH, VHO is the least efficient bulb in terms of watts/lumen output, and MH is the 2nd best (PC is 1st). But, in the case of all the other bulbs, the wattage is spread throughout the length of the bulb, if your clam will not be getting most of the wattage of the bulb. In the case of MH, the entire wattage of the bulb is in a very small area, giving a very concentrated and intense light to a smaller area, and in that area any light-dependant animal/plant will be getting lots of light. That is the main reason why MH is better, other reasons include fuller spectrum and more expensive =).


Absolutely no offence taken. I'm honored, actually.

I'm not sure I can fully answer your question, and I'm not sure VHOs are any less good than MHs per se, but two things come to mind:

  • A 110-W, 4-ft long VHO bulb spreads its 110 watts over a 4-foot length of bulb. a MH bulb is always at least 150 watts, and it concentrates that 150 watts in one small spot. This enables one to concentrate all "150-watts worth" of light directly over a particularly ight-dependent animal, something that cannot be done with VHOs.
  • It would take many more bulbs of VHO to match the light output of MH. How many bulbs do you have over your tank to result in 330 watts of VHO? I'm guessing three. My three
    250-Watt MH bulbs total 750 watts of light. In other words, MH helps one put more light over a tank by concentrating more wattage into each bulb.

Other than the first point above, watt per watt, I guess VHO and MH are equal. The two differences are that watt per watt, you cannot put enough VHOs under your hood compared with
MH bulbs. Also, with VHO, one cannot concentrate light so intensely in one spot.

Of course, the glitter lines are really cool. :D

According to an article in the most recent Scientific American, Fluorescent tubes are more effecient (lumens per watt). Don't have the magazine with me, but the had a chart with various types of light including MH and fluorescent. I seem to remember MH at 60watts/lumen and fluorescent at 80.

Other than the Scientific American article, I have not seen any studies conducted to show the relative differences in efficiency between different types of light sources. To me, a lot of what is floating around on these boards could very well fall into the "urban myth" category.

To confuse matters more, there is more than one way to calculate "efficiency". The three that come to mind immediately are watts/lumen, economic (Marine reef & fish 2001), PAR. Each method may give you a different answer as to which type of light is better.

If anyone has seen other studies, could you post a link (or source information) to it here.

My personal opinion is that it has to do with the glitter lines that HFF alluded to.
I don't know if there has been any studies on it, but I believe that the single point light source that a MH bulb produces, in conjunction with the refractive/reflective aspects the light source creates (glitter) travelling through the water best recreates the conditions found in the wild.
Just my two cents worth.