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MH Retrofit and UV lens shielding

I'm in the process of constructing a DIY MH Pendant.  My question concerns 
the use of either a glass or acrylic lens for this fixture.  My 
understanding is that nearly all single-ended MH bulbs available in the 
U.S. have a U.V. barrier as part of the bulb construction.  If this is true 
the only purpose of a lens would be to protect the hot bulb from moisture 
and possible explosion from rapid cooling?  I consulted a large commercial 
supplier of metal halide fixtures who had heard of this "exploding bulb 
theory" and commented by calling it "baseless in practical experience".

So I ask those of you using MH do you shield your bulbs and why?  Is it 
because it came from the manufacturer with a shield?  Those of you with 
retrofits or DIY units do you use a lens and why?  Do you use it on a 
pendant fixture or on a enclosed hood?  What material do you use or recommend?

Lastly, what are the implications of UV light slightly above natural 
conditions on fish and plants?

 From Andrew Lester                     atlester at home_com