Nice looking tank. From what I can see in your images it appears that your drop checker is usually blue-green. If your HC isn't growing under your current conditons then try turning your c02 up a little more, with the amount of light you have I think you could get away with it. Blast that C02 until your plants pearl and your drop checker goes yellow, however if your cardinals start reacting to the c02 levels you will know they are too high.
Just put up the bps to around 1.5. I'm guessing my cardinals will start going towards the surface if there's too much CO2?