I have a two peace lilly plants. Leaves on both plants are turning black. Starting at the edges and consume the hole leave. When black, the leave is dry and crumbly. I usually water once a week or twice if the leaves start to droop. I use well water (nitrates could be high). One plant is about 3 years old and the other is less than a year. The older one may be root bound, but he younger one is definately not root bound and the younger one seems to have the problem worse. I've been fertalizing once a month with miracle grow (diluted) to see if it helps, but doesn't seem to be. Both get good light, but not direct light. Neither are exposed to heavy handling. Any suggestions?