Transporting and replanting problems? Thats one thing I can think of; however, there is another instance... In college, my girlfriend and I had a 10 gallon tank that had only wisteria. It's one hell of a plant, we didn't add any additives to the tank and it grew like wildfire. I did notice that wisteria has a limited lifespan. After about 6-8 months, after it's reached it's most beautiful peak, I did notice that my plant did exactly what you're describing.
gah! Maybe I should take some tops from my 30gal? That one is doing PERFECT (if not too perfect heh) Ill bring a bundle from the home tank that is nice and fresh and i spose that would be a good way to test for deficiencies and things of the sort?
might be time to clean it out and do some replanting