Planted Tank Guru
Join Date: Mar 2009
Location: Madison, Wisconsin
Fluoride can cause black/brown leaf tips, though it more often happens in plants will longer strap-like leaves because the long leaf span concentrates the fluoride at the tip. I would be surprised to see it in H. polysperma.
There are a few other causes for leaf tip browning. I think some nutrient deficiemcies can also account for it but I can't remember which.