Mini nana roots turned black

[octopus]

I have a few mini nanas tied to driftwood and rocks and positioned on low light area of the tank. Yet they all seem to succumb to GSA. Even the roots have turned black from white. However the other plants (Hygro Difformis, Tiger Lotus, Mini Fissidens and Monte Carlo) have no traces of GSA). Not sure if the sudden GSA bloom could be a result of reduction in the plant mass as the problem only surfaced close to a month after trimming of the plants. Can certainly use some advise to contain the problem.
Temp : 28C
Fertiliser Dosage : EI
Water Change 20% weekly
CO2 : 3bps : 8 hrs (1pm to 9pm)
Lighting : 39 watts ( T5 tube almost 3yrs old) : 8hrs (2pm to 10pm)
Lifestock : Cardinals and Amano shrimps.

[Jimmy]

T5 tubes should be changed out every 9-12 months...