Two things make this tank possible: a pH monitor/controller that controls the C02 injection, and two 175w metal halides. I set the pH for 7.3, and the CO2 pummels our extremely hard Lake Erie water into submission 8-) Pure Cleveland tap water stabilizes at 8.4 pH. Trying to use CO2 to keep the pH down in Cleveland tapwater results in over 25 ppm C02 trying to keep it down to 7.3.
This tank is almost all RO water, resulting in around 15 ppm CO2 with a pH stable at 7.3.