70w Metal Halide on Nano

70 is the smallest as far as i know. If you have it high enough off the tank (like around 8 inches at least i'd say) then you should be fine. Make sure you got a clip on fan to blow across the top of the water though, cuz it will more than likely add heat.
 
I've built a canopy and stand for my tank, so I was thinking of installing a couple fans in the canopy, (2) 3" fans.
 
I had my 2x 70w 14k MH w/ 2x 18w super actinic PC over my 5.5 for a while my 15g+15g was cycling, it was f-ing great. I say put 2 MH over it. I just had a fan that clicked on with them and blew across the light. ATO is nearly necessary, evap is more signifigant with fan and MH, ATO is super easy to make though, and super cheap.

I am putting a 400w MH 20k over my 15g later this week when my bulb arrives, and selling my 2x 70w 14k MH w/ 2x 18w super actinic PC fixture.

You will love your 70w ers. MH is the way to go, everything looks better and grows alot better, it makes everything easier. I say get a 14k it looks great and your corals still grow. 20k looks better IMO but growth rates drop off, especially w/ 70w, low par. 10k is great for growth but looks a little to yellowish white for me, 14k is the perfect crisp blue light.
 
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