Dave Millman wrote:

Keith,

Buy one new one, plug it in next to the others, and compare. You'll end up
buying
the others too.

Yes, your old ones are still burning. But no, they aren't nearly what they
were
when you bought them. 18 months is about right.

Dave I am sure that is true. I guess what I meant by "still going strong" was
based on actual plant growth.
Fluorescent bulbs absolutely grow dimmer over time and problems can arise from
diminished output, such as increased algae and poor plant growth.
It just doesn't make sense to replace a CF bulb that is performing well.

Best regards,

Keith Moore