metal halide... what color spectrum?

[OrionN]

The crux of your argument is that MH bulb shift in spectrum as the bulb age and the shift cause the light output to favor less desirable algae and cause problem. This is fundamentally wrong.
The emitted spectrum of various metal vapors are fixed. It is the characteristic of a metal due to the configuration of it's electrons distribution. You can easily look up the spectrum of various metal on the internet. When I was in school umpteen years ago we had lots of fun in our physic labs ID various metals by analyze theirs spectrum. It have to do with quantum mechanic. As the electron absorb energy ti get boosted to a higher energy orbital state. When it fall back to a lower state it emit the energy in eletro-magnetic waves (photon). As the bulb age, some of the metal vapor get embedded into the glass (discoloration of the glass in old bulbs) incorporated into the electrodes and thus there is a net loss in the metal that produce light thus less light output. The spectrum of the light bulb remain the same but intensity decreased.
You can easily find information on Metal Halide light on the internet.

Thats not why nutrients is the problem, yes its the food, but one of the excess nutrients "predators" if you will, has been removed from the situation so you get an outbreak in population of the other "predators," the "bad" algae.

???I am not quite sure of your mean here???


In our tanks, everything grow as fast as it rate limiting needed allow it to grow. In a normal reef tank, the rate limiting need is not light but the essential nutrient that various organism (algae or otherwise) needed. That is why we have algae bloom in a new tank and the tank progress from one type of algae to another until most of the essential nutrient get "lock-up" in the biomass. There is no way we can keep a reef tank without herbivores. Even in nature, when the population of herbivores died like the urchins crashed in Florida in the late 80"s. The reef will be overtake by algae in short order. You can read about this if you want to do a search on the internet.

Sorry about the sidetrack of this thread

[StevenSeas]

From Bob Fernner on WWM
http://www.wetwebmedia.com/plantedtk...ightingags.htm

A note here on Spectral Shift. As they are used, all the differing light technologies change to known degrees in the percentages of what wavelengths they produce. Their luminosity decreases also, along with the preponderance of the desired spectral bands

You are correct that metals along with other gasses all have a signature light pattern that they put out. 100% undeniably this is true. However what isnt taken into account in your arguement, is a point that i made in the first post, "That as a bulb ages, different gases and metals leave the bulbs at different rates." This is a well known fact. That is what causes the shift, is the ratio of different metals and gases in the bulb changes, as some escape faster than others.
It is this spectral shift that favors "undesirable" algae.
Also I am not debating the fact about a new tank getting algae blooms, this debate pertains to spectral shift, which occurs to a detrimental point after about a year or more.

To OP, sorry about the shift of this topic, but I hope you can find the information useful for the future when it is time to change your bulbs. Also I hope that you are happy with what you ordered, please post pics so people in the future can use your experience for help in their own bulb selection.

[OrionN]

I would not trust this paper by Bob Fernner. He is clearly out of date in this paper. All you have to do is read what he said about MH in it.
Rather than this I would direct you to the many papers by Dr. Sanjay Joshi that can be found in the link below. Sanjay is an avid reefer and an Doctor of Engineering and a professor at Penn State. He extensively tested and wrote about MH lighting. Any serious aquarist who use or contemplate using MH light should spend sometime and read these articles.
Essentially according to these test, MH output decrease with time at all wavelenght. None of the wavelenght increase in intensity. Higher frequency wavelenght tend to decrease a little faster. At worst bulb, the intensity of light decrease by about 30% at two years. Anyway, I encourage the OP and anybody who follow this thread to read these series of articles, original publish by Aquarium Frontier and Advance Aquarist.
The major limitation of these studies is that he use different bulbs of the same brand at various age, not the same bulb at various age. Apparently there is significant variation from bulb to bulb of the same brand. Reading these articles help me a lot in determine which bulb to use and how often to change them. Read them and draw your own conclusion. Happy reading.

Here is the link:
http://www.personal.psu.edu/sbj4/aquarium/articles/

[ErikH]

http://www.manhattanreefs.com/lighting