Yellowing older leaves

• pH:
• Temp:
• KH:
• GH:
• NH4:
• NO2:
• NO3: 5
• PO4: 1.25
• Iron:
• Dosing: 5 mL Flourish, 0.5 ppm iron chelate, 13.75 ppm NO3, 0.8 ppm PO4, 1 tsp CaCl2, 0.5 tsp MgSO4
• NO3:PO4 Cumulative Dosing Ratio: 31.75:2.85 = 11.1

Notes:
The bottom older leaves of the L. repens have been turning yellow with black spots. It might either be a Nitrogen, Magnesium or Potassium deficiency. I think I've been adding sufficient potassium and nitrogen over the past few weeks, but I've been a bit low on the Magnesium, so I will try to increase it a bit.