[houseofcards]

I have 2x 175w MH light on a 90 gal tank. I currently have both lights on for 7 hours a day. I just set it up 2 days ago and have never used MH lights before.

What duration should they be on for? I don't have extra PC lights at the moment so the MH lights will probably be the only light on the tank for now. Will having MH lights only be a problem?

Personally I would run one for 6 hrs and both for only 1 hr intially and slowly move it up to 8 hrs and 2-3 hrs for both. It's all relative though to the amount of plants, maintenance, co2, etc.