how high should MH's be from the tip of the tank

[mojojojo]

how high should MH's be from the tip of the tank?

im running a 75g, (18" in height) 2x250watts 14k, the fixture is approx 7" away from the tip of the tank(the bulb is around 10" away from the tank) is this too low?

im keeping SPS and acans.

[FOSELONE]

10-12"

[Frick-n-Frags]

that's fine. if you dont have splashy fish you can go lower on deeper tanks, but 250w->18" tank is good at 10" above the waterline.

[reefscape15]

Anywhere from 8"-18" off the water. Higher up=better spread and less heat being introduced into the water. There is very minimal light loss from having to travel through the air, so don't worry about loosing PAR.

[mojojojo]

good stuff, so i should raise my fixture 3" more from the tip of the tank to tip of the fixture, to be honest im confused where you base on how high the MH should be, should it be X" from the tip of tank to the tip of the reflector or X" from the tip of tank to the tip of the bulb....

[fatdaddy]

good stuff, so i should raise my fixture 3" more from the tip of the tank to tip of the fixture, to be honest im confused where you base on how high the MH should be, should it be X" from the tip of tank to the tip of the reflector or X" from the tip of tank to the tip of the bulb....

If you're not having heating issues, then no worries.... I never thought about because my corallife has a legs, but I guess it's about 8". I use a fan during the summer months to blow the heat off the water.

[lordofthereef]

10" is ok. IMO it shouldn't be any lower than 6"

[Frick-n-Frags]

Anywhere from 8"-18" off the water. Higher up=better spread and less heat being introduced into the water. There is very minimal light loss from having to travel through the air, so don't worry about loosing PAR.


you actually have stated why you will lose par: better spread. light intensity falls with the square of the distance. IF your reflector has a focus pattern, then you can go higher without PAR loss. otherwise, yes you get PAR loss the higher you go.

[Frick-n-Frags]

mojo, just go with the lower edge of the bulb and the water surface. it isnt that critical anyway.

[bertoni]

I've run fixtures 3-5" from the water to the bulb without problems. I did have to clean the glass cover fairly frequently due to salt spray, though. Moving the unit up will lower the lighting level in the tank, reduce heat, and reduce spray hitting the fixture. Moving it lower will do the opposite. At some point, the fixture might be too tightly focused to cover the tank well.

I never had any problems with heat, so I tended to run MH fixtures close to the water.