In article , Richard wrote:
Well, there's one way to find out for su get one new one and compare it
to the old ones. The anal retentive and wealthy re-tube at 6 mos. I just
use twice the number of tubes and replace them when they burn out.

It would seem to me that's not a very cheap solution, the extra
electricity costs will outweigh the replacement costs of the bulb...

The 6-month guideline is generally only for super-light hungry reef
tanks (which I also keep) - and even that is up for argument. In
my planted tanks I replace my bulbs about every year or so, by then
there's a significant -and noticeable even to the naked eye- drop
in light output. My plants sure do grow like crazy after a bulb
change.

-Eric