As the field of ocean acidification has grown, researchers have increasingly turned to laboratory experiments to understand the impacts of increased CO<sub>2</sub> on marine organisms. However, other changes such as ocean warming and deoxygenation are occurring concurrently with the increasing CO<sub>2</sub> concentrations, complicating the understanding of the impacts of anthropogenic changes on organisms. This experimental aquarium design allows for independent regulation of CO<sub>2</sub> concentration, O<sub>2</sub> levels, and temperature in a controlled environment to study the impacts of multiple stressors. The system has the flexibility for a wide range of treatment chemistry, seawater volumes, and study organisms. Control of the seawater chemistry is achieved by equilibration of a chosen gas mixture with seawater using a Liqui-Cel<sup>®</sup> membrane contactor. Included as examples, two experiments performed using the system have shown control of CO<sub>2</sub> at values between approximately 500 and 1400 μatm and O<sub>2</sub> at values from 80 to 240 μmol kg<sup>−1</sup>. Temperature has been maintained to 0.5 °C or better in the range of 10–17 °C. On a weeklong timescale, the system has achieved variability in pH of less than 0.007 pH units and in oxygen concentration of less than 3.5 μmol kg<sup>−1</sup>. Longer experiments, over a month in duration, have been completed with control to better than 0.08 pH units and 13 μmol kg<sup>−1</sup> O<sub>2</sub>. The ability to study the impacts of multiple stressors in the laboratory simultaneously, as well as independently, will be an important part of understanding the response of marine organisms to a high-CO<sub>2</sub> world.