Excess Phosphate.
Symptoms:
Production of the Iron Phosphate causes the leaves to turn Black or Brown and to die. In addition if there is an excess of Nitrates your tank will experience an Algae bloom.
Cause:
Not performing your water changes on schedule or not changing enough water.
Remedy:
Change a large portion of your water right away. Then modify your tank maintenance routine to include larger water changes or more frequent ones.