Light dilemma

I have recently bought a 50 gal tank. Upon getting it home, I find that it is
not, as I had thought, 36" long but 35". It has tightly fitting glass covers
level with the top edge.. As I cannot put 36" bulbs under, can I lay them on
top, and by how much would the light be attenuated by the glass which is 1/4"
thick, and some distance from the water surface. I could fit 24" tubes under the
lids. Would the losses from putting them on top outweigh the difference between
24" and 36" tubes. As this is a very tall tank, with a not so big surface area,
to get any where near 3-4 w per gal, the tubes of whatever length are going to
have to be crammed pretty tight. Surely, if this is done less than half the
light produced by the tubes will get to the water, as that produced by the upper
section, reflector or not, will not be able to get back past the tubes to the
water? Thanks for your comments and advice,
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