I think it is probably a macro nutrient deficiency. Flourish (Comprehensive) only provides micros in decent amounts. Especially since you are using CO2 and decent light it is probaby starving. It probably fed off your substrate nutrients intially, and when that ran out, that's when it went bad.
Start dosing Nitrogen, Phosphate and Potassium, using the dry salts would be cheaper, and see how it goes. You can also use the Seachem stuff but it is a bit expensive.
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