Firstly, tank looking fire hot. Polys lookin happy from poly-mommas care. Job well done.
As for the black on the tail edge. It's part of the healing process, or growth from others biting and grabbing. No need to worry.... It's not the beginning stages of lepracy. Eel as for the plants. Call 911. Have no clue ;--) The polys are healthy at least. :---)