Tungsten Halogen

I have seen some of these lights (for illuminating driveways etc) and am
wondering "why not?" I can buy one of these units for 9 (~$15), whereas to
equip my tank with flourescents, ballasts etc would cost me well over 100
($150). Is there an "absolutely not" type reason?
(apologies if this is posted twwice, it did not turn up my server, although it
must have got somewhere because I got a reply!)
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