You should have just dosed around 1.5x the recommended daily dosage of Excel and still get results on the BBA. It just takes longer (about a week) but it is very safe for your fauna.
Anyway for your CO2, it isn't only about upping the injection rate. You need to have consistent levels throughout the day. Take different readings throughout the day, [perhaps on a Sunday] take a reading in the morning, once in the afternoon and another in the evening and compare the results of your CO2. Take the readings after you finished your Excel treatment to avoid any unnecessary wrong readings.
The circulation could be the problem. Your CO2 is not distributed well across the tank and that causes either unstable levels or simply deadspots. Algae drops in when such environments are such.
I am not good with equipment so I am not too sure if your current configuration is really optimum.
Studying the water chemistry and nutrients is a science.
Knowing what plants to get and how to use them is an art.
Aquascaping is a marriage of art and science, the logical and emotional.
-Calvin
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