Heat, efficiency and power for Metal Halide (MH) setups.


Lifetime Charter Member
Lifetime Member
May 4, 2006
I am going to experiment with my first MH (HQI, what does that stand for again?) setup. I purchased a double-ended 250W lamp with ballast from "fishneedit" on eBay to start with.

  1. How much heat would this 250W bulb likely generate? What about 70W, or 150W? (Not sure how to qualify this, actually, other than subjectively...)
  2. Are the fixtures, ballasts and/or bulbs heat sensitive? (That is, if there are hot days of 90+F where I am, will this cause issues with the light itself?)
  3. Would the 250 (or 150) be "ok"/"too much"/"just right" to use over a 36x24x24 (LxWxH) or 36x24x16 pressurized co2 tank?
  4. Are there any good rules of thumb to keep in mind when first using an MH setup?

I realize that the height can be raised or lowered to adjust PAR levels. What is a good starting height?


Plant Guru Team
Lifetime Member
Sep 23, 2007
South Florida

HQI = Hydrargyrum quartz iodide. Hydrargyrum quartz iodide - Wikipedia, the free encyclopedia

1. The lights will get hot. You can feel the heat when you get close to the light (be careful). I do not have specific measurements, but MH get hot. Ensure you have a fan in the unit. A 250 watt is VERY BRIGHT. You may want to go with a 150............

2. I live in FL and this has not been an issue for me.

3. You should be okay with a 24" deep tank. MH are very good for depth, not so much for a good spread. Will be better with C02 IMO. Again 150w may be better here.

4. They are very bright. Not sure what you use now, but be prepared. More light = more nute need/uptake.

5. I have mine at least 11-12" above the surface. 3X150w at 6500K for a 72x24x24. That's 2.5 wpg. Most likely more than I need.

Hope this helps.


Junior Poster
Feb 14, 2007
I use a MH over my 40 breeder (that's what is over it in the last vid i posted) It's a 175w 6500k. I've found that 12-13 inches above the water works best for me. I would reccommend hanging with chain so its easier to adjust if need be.