Being rich in Docosahexaenoic acid (DHA), the microalga Isochrysis aff. galbana (TISO) is regarded as a good live food for rotifer, copepod and oyster larvae, and as an indirect food for grouper larviculture. In order to resolve the insufficiency of CO2 with high-density algal culture, the effects of pH adjustment or CO2 addition for small volumes (0.5 or 1.0 L) of stagnant culture and large volume (100 L) of aerated culture under continuous illumination were studied. For the small-volume culture, CO2 or HCl was initially added to adjust the pH value to 5.5, 6.5, and 7.5. The growth was best for the CO2-6.5i group, while it was worst for the CO2-5.5i group. In the daily pH adjustment experiment, the CO2-6.5e group had the best performance with 221% proliferation compared to the control. The lowest cell density was observed in the culture with 1.0 g/L NaHCO3, and no significant difference was detected among the other groups. The addition of 0.5 g/L NaHCO3 and the adjustment of the pH to 6.5 with HCl resulted in 212% proliferation in contrast to the control. For the large-volume culture, CO2 gas or an HCl solution was used to maintain the pH at 6.5, 7.0, or 7.5. The addition of CO2 was more effective than HCl. However, there were no significant differences among the groups to which CO2 was added. The cell density reached 1.35 x 106 cells/ml, which was nearly twice that of the control on day 7. This study revealed that the addition of the carbon source can enhance the growth of I. galbana.