I have a 48x12x15 tank which is illuminated using a 36" white fluorescent
and a 36" tropical fluorescent. The water is medium hard.

That's 60 watts over a 37 gallon tank: 1.62 watts per gallon. A little low for
Cabomba, though your tank is only 15" deep, so it might be okay.

Is it normal for this to happen, and how could I get a plant to root
from a stem cutting?

IME, no, it's not normal for stems to die and rot.

As for getting it to root, it's hard to stop Cabomba from rooting. It will
spontaneously grow roots along its stems. You can cut it anywhere along the
stem, plant the pieces, and it will root.

And in regards to your other post...I've never had my swords die back in the
winter.

Could it be that the changing seasons mean your tank is getting less natural
daylight than it used to?


Leigh

http://www.fortunecity.com/lavender/halloween/881/