I have a 110 gallon reef tank being lit by metal-hallides (2x250w) and blue 4 actinic T5s. My light schedule is T5's starting at sunrise for two hours, then MHs for 8 hours, then T5s for two. Obviously the lights get very hot and I'm trying to reduce the water heat as much as possible; they're currently 10.5" away. I know LEDs would be better, but they're not in the cards right now.

The question is: How high can I raise the lights while still being effective for photosynthesis (penetrating 31" of water)? Is there maybe a 4'x2' hood that could possibly focus the light straight down, allowing me to lift the lights higher?

Also, if you have a recommendation for an alternate light schedule, I'm open for suggestion.

  • what size is the hallide? Is it 4ft or 6ft..... 110gals come in a tall and long version too, which do you have. I'm going to try my very best to help you with this
    – Christy B.
    Commented Jun 9, 2017 at 3:00
  • And what corals do you have? And are your T5s covering the full length of your tank as is? I asked about the size because you mentioned 31" of depth and 4' 110gallons are 29" deep.
    – Christy B.
    Commented Jun 9, 2017 at 3:03
  • It's possible that my measurement was a tad off but both the lights and tank are 4' long. I've since moved on to LEDs, but I'd be happy to get this question answered if it would help someone else out.
    – Gary
    Commented Jun 9, 2017 at 14:47
  • For heat , you may consider another approach : Fans in the hood and / or, refrigeration in the tank..And , shortened exposure. Commented Nov 7, 2017 at 1:05
  • Do you have a local reef keeping club? At least in ours, a few members have PAR meeters they'll loan out to other members. That's the route I would try if possible. Side note, what you're describing is exactly why I switched to LED's. I was running my chiller even in the winter when running MH. If you're looking that direction at all AI SOL blue's are relatively cheap at this point. You'd need 2 - 3 fixtures depending on what coral you're keeping.
    – Jestep
    Commented Dec 13, 2017 at 19:46

1 Answer 1


Not knowing what type of corals you have (soft vs. hard) this is what I've come up with:
If you have a hybrid fixture (metal halides/t5 combo body fixture) you will want them about 6-7" inches from bottom of the bulb to top of the water (most of these that come with legs are 4-6" which is okay but not optimal). If they're two different fixtures (a better result for your reef) mh should be 9-10" from bottom of bulb to surface since you have 250s. The t5s 4-6" (you could theoretically do 3" but it's common it'll get too hot, even with a cooling fan), plus you want to keep salt splashing down to a minimum to extend the life of the bulb (usually lasts about a year in a good setup).

I don't see anything with your light schedule being 2,4,2 especially if you're concern is heat, then keep it that way...it's just more common to see 4,4 hour interals- mostly for convenience

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