Re: Aquatic Plants Digest V2 #530

Re the issue of utilizing a cover between a light source and tank water
surface: the historical argument against mainly deals with the loss of 1)
intensity through absorption, reflection and refraction through something
with a different optical density than air, and 2) spectral shift from the
same... not to discount the added effects of scale, dirt and scratches on the
cover... In practice, fluorescent lighting should be fine and optimized if
placed near (inches) the waters surface, bubbly or not, as long as care is
taken to keep the ends, pins and all covered (a little silicone grease and
those spiffy cover-all endcaps do wonders). 
     With metal halides, mercury vapor and some other more novel modes, this
is not the case, and one should take care to read and heed the manufacturers
recommendations and warnings concerning shielding for heat, far-end UV, and
the potentially disastrous effects of rapid differential cooling of said
lamps by splashing et al. Per spec. much of this lighting should affixed a
good foot (30cm.) above the water's surface, and periodic cleaning of the
transparent shields made part of the maintenance routine. Don't be foolhardy
re the use of the protective elements; their are unfortunately, several
records of serious hobby accident from lighting... houses burned down, bad
electrical shock, exploding MH lamps...