Well first I would advise against using any sort of pH buffers. They are highly unpredictable and more often than not, just prove to cause more problems than solutions. What kinds if plants are you having trouble with? Do you have a drop checker? Using bubbles per second is useless because numerous variables make co2 concentrations different even with the same "bubble count." Use a drop checker or your fish to gauge co2 concentrations. How are you diffusing it?
Is your water stained with tannins or a green water algae outbreak? Both of which can prevent light from reaching your plants with adequate force. Depending on what is going on will dictate how you handle it. If it's only tannins then keep changing your water. If you want softer water then you might have to use R/O or deionized water. If you have good co2 and diffusion, adequate light, plus fertilizers, you should see growth. Are you dosing ferts?