13 watt. So how big is your tank? And it is several inches above the water surface? It is likely that the light level is pretty low, and with tannin in the water that makes the light even dimmer. If the rhizome was buried, it would rot and the leaves would fall off, not turn brown. Low oxygen in dim light makes it worse as well. Try adding an air stone.
Bolbitis fern comes from fast moving and well oxygenated water. Moss prefers low temps. Tannin is not a problem if you compensate for it with light.
Robert Paul Hudson
RAOK Club #1
Kindness like love is unconditional