My Marsillea carpet is melting in my office tank and I am

Anyone with similar problem?

It does not follow a certain pattern. There are spots (some of one sq inch, some larger) with stems melting from roots upwards. Last Fri during the water change I have removed as much as I could from these melted stems and today morning I have discovered that the melting extends.

Tank's spec
Volume - 320liters
Light - 4 x 80w T5 HO - Philips 865
Photoperiod - 9Hours
Filtration - TetraTecEx1200 + TetraTecEx700
Substrate - JBL Peat, JBL AquaBasis, unwashed quartz sand all covered by AquaMedic Volcanit

Ph - 6.6 to 6.7
Kh - 18
Gh - 23

Ferts
Mon - Wed - Fri
KNO3 - 0.7tsp
K2HPO4 - 0.1tsp

Tue-Thu-Sat
15ml - JBL Ferropol

Fri over 50pct water change

Thanks beforehand.