Try doing a 3 day balcout.
Works as well as the antibiotics and cost nothing.
It also addresses the long term problem already mentioned, low NO3's.
Your NO3 kit is very likely off.
Add a little KNO3 to some water and try to see if you get any reading etc.
Don't add any KNO3 and see if you get any reading etc.
All you do is clean the tank out well, remove all the BGA you can.
Then do a 50% water change, add KNO3 to the tank and turn off the lights/CO2 for 3 days. Cover tank with trash bags. Make sure no light gets in.
Uncover the tank. Do another 50% water change, dose KNO3 back into the tank, and keep up on your dosings, (1-3 x a week generally depending on growth rates).
It's that easy, it's free and you do not need 15$ worth of drugs.
Also do you use CO2?
That's too much light without CO2, your pH is high(7.6) so I assume you are not using CO2.
Using CO2 will help the plants grow faster.
It will also increase the uptake or PO4 etc.