Would it be easier to give the filter flow rate in gallons per hour?
Water changes need to be increased nearer to 50%. This is because the added CO2 increases plant growth. As a result, the metabollic waste that the plants produce also increase. We need to remove these, hence water changes of around 50% per week. The waste could otherwise build up thus harming fish or causing algae.
Dosing Seachem Flourish every 3 days is fine however you also need to source some macro nutrients. Flourish is only good for trace elements. Macro elements are nitrogen, phosphorus and potassimum. Plants need these much more than trace elements. They especially need lots because the increase in CO2 increases the nutrient uptake.
I'd say you need to increase your flow/circulation. The filter doesnt seem to be enough. There are some really good powerheads out there that can provide flow. Hydor Koralias are good. As a guide, you want 10 times the volume of the tank turned over every hour (that's minimum for a tank with CO2). So, your tank would require atleast 113litres per hour or 300gallons per hour.
It's important that the drop checkers contain the right solutions. Using tank water is useless. It has to be 4dkh water and bromo blue reagent.