I think one of my Anubias rhizomes died (rotted) due to it being tied too tightly. The Anubias rhizome is supposed to be hard and green, if it is soft or brown, something is wrong.
For Anubias, the rhizome is the most important part of the plant, if it is dead, the whole plant will die.
Another reason for the Anubias rhizome rotting is the dreaded Anubias disease, but I don’t think it is the case here because another rhizome just next to the rotted rhizome is perfectly healthy.
Hence, the conclusion is that the Anubias rhizome most likely died from being tied too tightly by the string. It was at the back of my tank so I didn’t really notice it at first. I only noticed it when I saw a new leaf sprout out from the rotting rhizome, but the new leaf melted soon after. Usually, when new leaves melt, it is a sign of something wrong. Upon taking the entire Anubias plant out, I then realized that the rhizome had turned mushy.
The original Anubias was tied (by the seller) with thick string. I don’t really blame them for tying too tight because it is not easy at all to tie Anubias (too loose and it will not stay in place). I cut off the old string, threw away the rotted rhizome, and re-tied the healthy Anubias lightly with sewing thread.
Probably the same problem can occur for Java Fern, Bucephalandra, the rhizome may rot if tied too tightly.