70W MH - what should be the distance from water?

[dendro982]

70W MH - what should be the distance from water?
Safe and efficient, I mean.

So far it is in slightly less, than 5" (11 cm) from the water, and 5.5" (13.5 cm) from the topmost coral, porites. Had read, that others have such light at 7" (17.5 cm), is mine too close?

Thanks.

[Blown 346]

You would want the lights no less than 6 and no higher than 12 inches from the waters surface.

[tedu]

I've got a 12 gallon nano with 70w MH inside the canopy. That puts it about 4" from the water level.

No issues other than the obvious ones around heat.

[Rosseau]

It may seem like a total newbie thing to say, however I've never really understood why lights need to be a certain distance off of the water.

I can see these reasons...

-high enough that they won't be splashed with SW
-high enough that radiant heat from the fixture is not transferred (as much) to the water surface
-Better spread of light across your tank

Other than that...?

The amount of light reaching the surface of the water will not vary much whether the lamp is 6 or 12 inches from the water. Certainly some is lost by scattering/absorbtion in the air, yet I believe this is minimal in less than 12" of air.

Even light like UV shouldn't decrease much over that distance.. am I way off base here?

Is the only reason that you will have a better spread of light, and thus less intense W/m^2 at any given spot on the water?
Will 70W of MH really exceed wha the corals can take, even if the lamp is 0.5" away from them?

[jgiannini]

I have my 2 x 150 watt MH's about 3" above the water with 2 x 70 cfm fans cooling them and the water. My lights are on for about 6 hours a day with no temp movement.

[Spuds725]

High enough so they stay dry is the most important--

I'd target at least 6" off the water, 8" or more would be better. Mine are about 10" off the water.

[GoingPostal]

The Aquapod with 70 watt halide is maybe four inches off the water, no heat issues here.

[dendro982]

Thank you, most helpful!

As I understand, no significant UV danger even at my lamp height. Will place it slightly higher, at 6", just in case.