Do you have a drop checker on the tank? If not just keep nudging up the co2 untill you see stress in the animals. Also what is your lighting schedule? From the looks of the algea you may be leaving the lights on too long.
No drop checker yet. I do like the ones they have at GLA though. Surely I'll get an itch to get one before long.
Right now, I have C02 flowing at a rate that keeps the PH steady between 6.4 and 6.8. I know my GH/KH so I can get a ballpark number if I need.
My photo period is 8.5 hours right now. The tank is just now coming off it's cycle, so it could be something to do with the spike in nitrates in the water column. I'm doing daily (or every other daily) 20% water changes so I have faith that I will conquer it soon...unless I put soup into the water. Ugly Genius did!