how high should i mount my light ?

i have a 175 watt metal halide pendant. i was wondering how high i should hang it? do you measure from the water surface or the bottom of the tank?

i plan to keep mostly hard corals and few sps. thanks

 

It depends on what lighting you had in the past(stronger or lighter) than the last. Or, did you just set up your tank with the metal halide. Is the tank set-up already? Is it a new tank?
If it is a new increased lighting system upgraded from a lesser lighting system, I would set it up higher in the air and then gradually lower it to a desired height, taking into consideration the high temperature of the metal halide bulbs emmitted. A fan is a good choice to minimize the heat penetrating the water of your tank, causing a raise in water temp and temp. fluxuation. This is due to the fact that you don't want to stress out your corals causing too much oxygen in the corals tissue. This could prove fatal to your coral, as it causes an abandonment of zooanthellae and bleaching of your coral.