MH Canopy Design Question

[dr x 2]

I am currently in the design phase for the canopy for my 90 gal reef tank. Since I am new at this I have some simple questions that need clarification.

1) Should it be a totally enclosed design, or can you keep a space open on the back, or no back. I was concerned about ventilation, but with an open/partailly open back, wouldn't there be a lot of light coming out the back where the vents (or no back) are? I also plan on using fans, probably two 4"ers.

My current design is 14" high, (figure 6" for the retrofit) which puts me at about 8" from the water.

The vent space runs the length of the hood in the back, but only 1" down from the top and about 3" in height, so the reflector should deflect the light downward no?

Also with the lights that far away from the surface of the water:
1) do I need a lens?
2) Does it affect the lighting i.e.-increase the depth of the tank. I was planning on 175 watt MH x 2 with either VHO or PC suplemental lighting. Should I consider 250 x 2 because in theory, I am increasing the height of the tank to 30, and from what I read with a tank of that depth, 250 x 2 would be more appropriate for the organisms I want to keep.

I am snowed in here so I can't get to my scanner to get a pic. I hope I explained it well enough.

VERY CONFUSED! I am probably thinking too much, I should just build the thing.

Your help is appreciated.

drx2

[jason gilman]

yes you should leave the back out.
the reflector should conscintrate
the lite in the tank like its designed
to do .the more ventilation the
better.are you gonna use two fans,
and push air across the bulbs and
out of the canopy.a stainless mesh
would be somthing to cosider putting
across the back. :wink:

[dr x 2]

thanks, thats a good idea

[ryan]

you are right about your depth theory but you also have to think about this, air doesn't affect the light near as much as water does, so you raising the light up 8" above the tank is like your tank only getting .5-1" deeper