MH pendants and tank size

Anyone out there have a feel for the largest tank a *single* 175w MH
pendant can support? I mean to provide enough light for fairly
demanding plants. I know its easily enough light for a 2' long tank,
but how about 3'? (I'm not really worried about depth; all of the
tanks below are fairly shallow.) 

Here's the standard All-Glass tanks between 2' and 3' that I'd be
interested in:

Tank Size       Exact Outside Dimensions    
                (inches) (L x W x H)        
                (Including frame)           

20 High         24 1/4  x12 1/2  x16 3/4  
29 Gallon       30 1/4  x12 1/2  x18 3/4  
30 Gallon       36 1/4  x12 5/8  x16 3/4  

Any comments?

Dan Resler

Dept. of Mathematical Sciences            
Virginia Commonwealth University           
Richmond, VA                                    
23284-2014 USA
email: resler at liberty_mas.vcu.edu