I had noticed that in a couple of my tanks, the plants weren't doing very well, growing weakly and with leaves dying. This was rather perturbing as having densely planted tanks with healthy plants is one of the principles of keeping aquaria for me. Last week, one of the light tubes (T8) went and so I replaced it. When I switched the lights on, I was amazed firstly how bright the tank looked and second how much brighter the new tube was than the older second tube that was still in place and so I also replaced the second tube and the tank looked even brighter. I then went to the other tank that had poor plant growth and repeated the exercise with the same result. Within a few days, the plants have perked up considerably with new leaves sprouting everywhere. They need some pruning because they have become straggly, but I anticipate them soon being back to normal.
The moral of this story is that it really does pay to change your tubes before they lose too much efficiency, particularly if you have live plants, though the tank overall and the fish also look much better now. Of course, all of the literature tells us to change tubes every year, but this is something that is very easy to overlook. The lights dim very gradually and so it is easy to forget how bright they should be.
A reminder also that there's no need to pay fish shop prices for tubes, the much cheaper ones for general house lighting are just as good. There's no real evidence that tubes giving off a special spectrum are actually any more effective in freshwater aquaria. I stocked up last year by buying online for even lower prices and so always have spare tubes available.
I do have two tanks lit by T5 tubes which seem to keep their brightness much longer and don't need changing as often.