You appear to have a macro nutrient deficiency either or both nitrates and phosphates and I see leaf yellowing which indicates iron deficiency. With the lighting you have you need to be using Excel or Glut at 1ml per every 2 gallons of water. Then you need to be using dry ferts to get all the macro and micro nutrients the plants need. Flourish is pretty much micro nutrients.
Do you know your nitrate and phosphate readings?
You appear to have both the start of BBA, black beard algae around the edges of the leaves and I see some GSA, green spot algae on the leaves as well. The GSA will usually go away and not return when phosphate levels are at least 3ppm. The BBA needs to be spot treated with Excel or Hydrogen Peroxide 3%. Turn off filters, pull up 1-3ppm of either Excel or Peroxide in a syringe, slowly squirt the algae, leave filters off 20 minutes. Usually within 24 hours the BBA will begin turning red, pink, or what which means it's dying.
But until you correct the problems causing the algae it will return. You have good lighting but not enough CO2
/liquid carbon and definitely not enough of the right kind of ferts.