Tom has tested PC bulbs and as I recall NO bulbs. And he runs them all till they die. It seems that the idea of replacing the bulbs comes from the Reef people. And the spectral shift is very hard on the corals. Also remember that this "wisdom" is several years old at least and was formed back when keeping corals was a new and novel idea and just getting enough light to keep what are now considered "easy" corals alive was hard. So combined with the spectral shift and gradual loss of light it was hard on the corals.
New PC bulbs are much brighter and a bit higher in spectrum than older bulbs. But the loss of both is about 10-20% maximum during the life of the bulb with most of this loss coming in the first few months.
So yeah I would say it's pretty much a myth in the planted tank area that you have to replace the bulbs frequently.
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