My lighting(T5HO) period is 8 AM to 8 PM......I don't use CO2.
I'll add ~12 amano and some duckweed soon !!!!
The tank is not near a window but I'll check for phosphates, that's prolly the problem....
Plants need phosphates so I wouldn't really be overly concerned about that. You do need a source of carbon with that much light. I'd strongly urge you to do DIY CO2 and/or Flourish Excel. I agree that you should reduce your photo period to 8-10 hrs.