I've used tubes for years before changing, I typically only change them when they stop working. To me it doesn't really matter if they lose any brightness as long as the plants are growing healthily - I've never ever seen lack of light due to old dim tubes being a problem. Most tanks have problems due to
too much light. If your tank has no problems, providing more light will only increase growth speed (and nutrient demand). The difference between an old and new tube should be negligible for our purposes, even if it isn't, lower light will cause no issues.
That said, just like Ian I'm surprised to see the difference in your Fluval bulbs within 11months, this would put me off buying Fluval bulbs if I didn't already avoid them because they are over-priced. Maybe it isn't so surprising as Fluval themselves recommend regular changing so it would be to their advantage to have short bulb life spans. I've never seen such a difference with the Phillips, Arcadia and Sylvania brand bulbs I'm using in electronic ballasts now. I do notice such dimming in my T8 tubes on old magnetic ballasts...but as as mentioned I take no bother because the difference in PAR is negligible from a plant growth point of view.
If you want to frequently spend £££ on a brand name T5 tube for the sake of slightly more brightness then that's cool, but there are far more economical options to get more brightness, like investing in LEDs. Or even just buying normal T5 tubes:
https://www.ukaps.org/forum/threads/cheap-ho-t5-fluorescent-tubes-update-with-photos.555/
Well I guess the botanists are wrong too. You clearly know more than they do...
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If you believe your assertion that spectrum is important with regard to aquarium plant growth - maybe try to back this up with references (or even an just explanation of your own understanding) rather than attacking experienced members with sarcasm? This topic has been covered extensively and repeatedly on these forums though, so I would recommend you do a wee bit more searching and reading first
.
You are not totally incorrect with the statement, "spectrum is also important". I thought this when I saw OP's picture. We are only seeing the result of photons the cameras sensors have picked up, the brighter tube may be emitting more photons that are visible to our eyes or the camera sensor, but perhaps useless or of negligible use to to the plants.
Spectrum can be important when growing a single species of plant too, such as growing tomatoes or cannabis indoors. Tomatoes and cannabis can make use of a wide spectrum too and can adapt, so if your aim is to simply grow a plant, then the exact spectrum is not important at all. If your aim is to grow a certain dry weight of valuable cash crop working under very tight margins (like commercial production of tomatoes in greenhouses with supplemental lighting, or illicit production of cannabis in small indoor tents) then choosing the optimum spectrum of bulb for your particular plant species could mean the difference of thousands of £ profit, even if it is just 5% extra dry weight.
Even if it was possible to choose an optimum spectrum for our aquariums, we would only achieve slightly faster plant growth (or more algae), so there is less reason to wish for this.
However Ian is totally right as far as growing aquarium plants is concerned, there
is no optimum spectrum for aquariums and how it looks to our eyes is more important. We grow so many different species, each one would have it's optimum spectrum depending on where it evolved and what photosynthetic pigments it can produce, also plants can produce different pigments in response to different lighting conditions (such as plants producing red carotene pigments when exposed to bright light).
Tbh due to so much erroneous and conflicting marketing information it is not an easy subject to understand and nobody should feel bad about being confused!
)
. I guess in reef-keeping the spectrum and intensity of light is more important to Corals than a planted aquarium!
