Brown spots on java fern leaves

• pH:
• Temp:
• KH:
• GH:
• NH4:
• NO2:
• NO3: 5-10
• PO4: 0.2-0.5
• Iron:
• Dosing: 5 mL Flourish, 10 mL iron chelate (1 ppm), 10 mL KH2PO4 (0.4 ppm)

Notes:
I noticed a couple of the java fern leaves, including new leaves, suddenly have gotten brown patches all over, and some of the little plantlets on one of the leaves have gone all brown. They must have developed over the past couple days. I don't know what's happening there...