Yes, I use osmocot2 pills, but it is time to plant some more tomorrow. Some leaves do fall easily but it is usually only when the stems get crowded, h. Lancea will only keep leaves but so low no matter what it seems. I don't believe I have any up close pix right now other than what's on my journal, once I find my camera again I will try to post some, but colors, leaves, growth, etc.. all seem to be normal, this weak stem seems to be the only problem I have at the moment.
Sounds like it is not getting enough co2 and nutrient from your flow. Since all your plants in your tank are healthy. I would experiment by moving one of the stem to the front with no other plant is blocking it to see if it will be more robust. If it is, then you know your problem.