[Newt]

Rule of thumb is;
T12 and T10 every six months. I have a chart some where that shows the decline of the cathode tube.
T8 every year
T5 (including CFs/PCs) annually
T5HO dunno yet.

T12 actually can decline 60% in six to 12 months. Other more efficient tubes can go longer but the cathode tubes all decay over time. I'd never wait until they burn out. The intensity deminishes too much and the spectrum (spectral outout)can be altered. The Triton tube (a T10) by Interpet claimed that this bulb would not change in intensity or spectral output from day one until it burned out. Not sure how they are the only ones on the planet that can accomplish this.

If I come across the decline chart I will post.

 

[Newt]

Me, too.

A good alternative to the Aquaflora (as it is very difficult to find/get) is the Aqua-Medic Planta bulb.
http://aqua-medic.com/t5_bulbs.shtml
The bottom graph is the spectral output for the Planta.

 

[Newt]

It comes from knowing that all fluorescent lighting cathode tubes decay over time and the result is diminished intesity and in some cases a shifting of the spectral output.