Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
A sudden decrease in light or growth is easy to notice. A slow decrease over two years, almost impossible.
The replacement frequency for T5HO is situational, depending on the plants' requirements and original light intensity. Some may "need" to be changed more frequently than one year only because the light intensity was marginal from the start. Or if you have more wiggle room, less frequently.
I have a dimmable T5HO fixture, and didn't need max intensity from the start. Wiggle room galore. Although my bulbs are 1.25 years old, I've been able to compensate so far by simply turning them up. And I have a spare bulb so far used only to swap in occasionally to see how much the old ones have degraded, which is definitely noticeable when the new is compared against the old. I think they'll need to be replaced around the 1.5 year mark, as I just had to turn them up to 100% to maintain the intended high light.
At two years, your bulbs are probably producing half the usable light they originally did. If you were originally running a high light tank, you're now running medium light. It may not be the cause of your BBA, but I say it's time to change the bulbs regardless!