Rotala rotundifolia stem breakage
I have a lower light tank that I fertilize every other day with the Macro/Micro nutrient mix, plus extra potassium.
Most of the plants are doing well, but my rotala rotundifolia is experiencing upper stems that get soft and black, then break off without warning. These aren't the base of the stem. They're halfway up. Ditto for this large feathery plant that usually grows like crazy. Even the larger rooted bundles are just disintegrating.
Meanwhile, the java fern, anubias, moss, melon sword, crypt retrospiralis etc. are growing just fine.
What's going on?