Metal Halide - Choices
 

After reading the following article http://www.advancedaquarist.com/2010/1/review1 I started to question what is the most efficient way to light a 6ft tank.

At present I have 3 X 250W MH which give me good coverage but I have seen many with 2 X 400W MH on the same length tank. It seems that you would get more over light but maybe not spread as evenly.

Any ideas?

What exactly are you asking? Efficiency for light vs. cost of running them? Or overall output for benefit of corals?
Personally I'm going to run 3x400w over my 6 foot tank, as I don't feel that a single bulb will cover 3ft adequately.

From the article though, those bulbs will be a consideration when replacing my radiums.

Based on pure numbers 3 250W halides seems more efficient than 2 400W halides. The type of ballasts might change this but really I wouldn't see the gain. Spread is based more on the reflector over the wattage so if you wanted to go with 2 400s you would want reflectors that light up a 3' square well like a lumenarc but then your tank isn't 3 feet wide so it doesn't really make sense. For halides you're probably better sticking with 3 250s, if you want to increase efficiency look into different reflectors as well as bulb and ballast combinations.

I agree that keeping the 250s would be better than switching to 2 x 400. The difference is probably not enough to warrant upgrading to 400 3 times.
FWIW, I was going to use 250s, but a screw up on my bulb order a couple of years ago put me into 400s (again). Now with a deeper tank, I'm sure they'll work better, especially running radiums.

Fun fact, 3 400W halides running 12hours a day will cost close to $40 per month in electricity!

What I am reading in the article though states that you get more light for less power out of two 400 vs 3 @ 250.

How is 2x400 less power than 3x250?

The 400s are going to burn 800 watts and the 250s 750 watts?

I'd stick with 250s myself. I know it's a dying breed but I've finally tracked down another hqi ballast for my tank. I can run my radiums as they were intended and down the road I can run a lower kelvin bulb and get super sweet par numbers.

That is what I always believed as well but you must take into account the power to drive the ballast as well as the amount of light they give off. So for instance a Vertex 400 watt ballast actually consumes 444watts with a ppfd of 121 so at 888watts you get 242 ppfd. Watts per ppfd 3.6

A vertex 250w consumes 263 and produces 52 ppfd for an output of 789watts at 156ppfd. Watts per ppfd 5

So for marginally more power consumption you are getting considerably more light. That is why I was questioning it. Maybe with 2 400 I can run the lights less consuming the same amount of electricity and get better results.

Does that make sense or do you loose out because your are not getting as equal of coverage with only two. I also think that I don't put much in the last 4-5" on the extreme sides of the tank so I can still clean the glass. Last consideration is that 2 400W bulbs are less expensive that 3 250s.

- Joe

I am thinking that 3 of the lower wattage, will add up to better coverage overall. While providing a more evenly spread of light.

2 of the higher wattage would be a smaller footprint, and give you hot spots.

Keeping in mind I didn't even glance at the article, what I think they mean is that you get more intensity out of the 400w bulbs than you do out of the 250w bulbs. You use 50 watts more using 2x400w than 3x250w, but the intensity you get out of the 400w bulbs makes up for that. So essentially if you get more intensity for close to the same amount of electricity, the efficiency is greater using the 400w bulbs.

Having said that, I still think you're better off keeping 3x250w over a 6' tank unless you get some sort of custom reflector for 2x400w that spreads the light over the 6' length, but doesn't spread it past the width of the tank. Now that would be interesting...