I think you have a tank with a lot of potential but needs a bit of a refocus. For CO2, I think you would really need pressurized CO2 with a HUGE reactor to get it to lower your pH for an extended period of time. The CO2 block system just does not work for tanks this large.
I am not sure if your aim is to recreate a biotope or a planted tank with South American fish species? If your goal is too recreate a biotope, then I would pull the plug on the CO2, decrease the amount of lighting, weed out all the crypts/ferns, and grow only Echinodorus species which are tough enough to survive the digging of the Geophagus.
Otherwise, I would remove the Geophagus, upgrade your CO2 system, and start learning about fertilization. There are plenty of great plants AND fish for a nice South American themed planted tank -- and you have a tank large enough to effectively use Echinodorus sp (sword plants)!