Ocean acidification is one of the phenomena caused by climate change in the world. Most Ocean acidification (OA) experimental is carried out in a laboratory by creating a simple system. In this research, the seawater in tank is given CO2 gas regulated using a needle valve driven by stepper motors using Proportional-Integral-Derivative (PID) controller. This tank is equipped with a stirrer to accelerate the system's homogeneity, flow sensor, and pH sensor. Arduino as a system control is tasked to regulate stepper motors, reading and keeping the pH value according to the set point value while recording the changes of pH value occurring in the tank. This research shows that the system can reach a desired set point and maintain it with 1.7 % overshoot. From the experimental results, OA system has a good response with a final pH value of 7±0.09 that can also be concluded this system is very suitable for long-term research.