This article is from the Aquaria: Plants FAQ, by multiple authors.
Most fluorescent bulbs lose a major portion of their intensity after
six months, so they should be replaced every 6-12 months (T-8's can be
kept longer). If that seems expensive to you and you can live with the
reduced light level, you can cheat and wait until the bulbs burn out
after two years (that is, according to TAG editor Neil Frank, what
"many experienced plant enthusiasts" do). It is best to stagger the
replacement on multi-bulb tanks in order to avoid dramatic intensity
changes.
 
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