From: AAronson at aol_com

>I am thinking of switching from my 3 20W fluorescents to a home brew halide
>lamp.  For a 37g tank is one 175w 5500 bulb enough?  If so, how would I get
>the light to evenly disperse across the surface of the bottom of the tank, or
>should it not matter?

175 Watts should be plenty. 

MH kits usually come with a 2'x2' reflector that can be retrofitted
into an existing canopy.  But it could be simpler for you to use a
pendant fixture.  

I believe there is a lot of material on DIY lighting archived at
Caltech.  Why don't you take a look?