MH Lighting Question

[jgb23]

As I dig deeper into my tank and the posibilities, specifically around lighting, I am at a crossroads and was hoping for some direction. I have two 175 MH for my 75 gallon tank. For the time being, I am pretty happy with 350 watts, which is approximate 4.7 W per gallon. I eventually would like to supplement deep actinics with 110w of PC.
Today while at my LFS, the shop keeper introduced me to 15K bulbs. Since this is the halfway point between 10K and 20K, it really gives a nice color to the tank. My earlier thought was to go with 10K MH and suppliment actinics using PC. After seeing these bulbs, I believe I can get good color and spend the 200 for PC on something else (pumps, etc, live rock, etc). I am just not shooting for color, I want to give the animals the appropriate lighting they need as well.

Here comes my questions:

1. Is there any downsides to going this route? Is there any corals that will not flourish in 15K?

2. From what I read and what people have suggested, the concept of how much light is enough always seems to elude me. I have had someone tell me that 2X250 W MH is plenty of lighting for a 75, yet another say that 2X250 is overkill and that 2 X 150 would be plenty. I went with 175. Is there a rule of thumb when it comes to lighting? I have read that you should have anywhere between 3-6 W of lighting per gallon. I want to find the yin and yang or balance on this lighting.

Thanks in advance for any advice you give me.

Best Regards,

Joe

[charlie]

WPG is not a good way of determining anything, what you are looking at is how much your lite will penetrate the water.

http://www.reeflightinginfo.arvixe.com/articles.htm

That link is to a bunch of articles by Sanjay Joshi. That ought to about fill up your nite.
If you can't find it there, you can't find it.

[Poseidon]

The watts per gallon thing is USELESS! It fails to account for the shape of the tank. Think of the difference in a tank that is 100 gallons, but one is 30" tall and the other 18", do you think the light at the bottom of each will be the same if they both have 500W of total light?

Actually WATTS has very little, if ANYTHING to do with light, you want LUMENS, or in this hobby, PAR, or PPFD.

The link Charlie gave you is the WINNER, if it is not listed there, DON'T BUY IT!

[Rubisco]

The watts per gallon thing is USELESS! It fails to account for the shape of the tank. Think of the difference in a tank that is 100 gallons, but one is 30" tall and the other 18", do you think the light at the bottom of each will be the same if they both have 500W of total light?

Actually WATTS has very little, if ANYTHING to do with light, you want LUMENS, or in this hobby, PAR, or PPFD.

The link Charlie gave you is the WINNER, if it is not listed there, DON'T BUY IT!

Lumens are USELESS! pay attention to PAR (actually the proper way to do it is measure in Quanta unfortunately most of us dont have the money to purchase a Linear Quanta meter), the problem with lumens is that it is a measurement based on the human eyes' perception of light wich interprets heavily in green.
PAR is the actual representaion of what drives the Photosynthetic response.
I personally dont feel that WPG is all that useless just more of a weak guideline that can be use in conjunction with PAR to assist one in a lighting decision.I think most of us know that anything deeper than 24" is going to require a minimum of a 250W MH, at least it seems common knowledge to me.

JOE, I too have a 75 gallon reef. Ive got 2 175w MH and 260w of PC over the tank.
I currently keep a variety of corals and inverts primarily SPS and Zoos I also have two clams.
FWIW


Matt