Hello everyone; My plant has decay from top to bottom. What is the reason? What can I do?

Looking for some stems for a red accent for my tank. Have plenty to trade - Anubias Nana, Anubias Nana Petite, Needle Leaf Java Fern, Amazon Sword, Staurogyne Repens, Water Sprite, Amazon Frogbit, some Crypts, etc. Please email me (just choose email from the drop down when you click on my...