The value of macroalgae in a healthy human diet is becoming increasingly recognized and supported throughout Europe. Macroalgae provide a rich source of vitamins, minerals, proteins, fatty acids, and antioxidants that also support the functionality of macroalgae in other industries, including cosmeceuticals, pharmaceuticals, and more recently, packaging. Sustainable aquaculture of macroalgae will be necessary to supply the increasing demand for macroalgae as a functional material, considering that natural harvests are limited and cannot keep up with demand. Different methods can be used to cultivate macroalgae, including flow-through systems or recirculating aquaculture systems (RAS) with natural or artificial seawater. The latter provides strict control over the growth conditions and water quality in order to provide a high quality and traceable product. Additionally, environmental conditions such as salinity, temperature, and light can be modified to optimize the concentration of functional ingredients in macroalgae. While most research efforts have focused on seasonal and geographic trends in concentrations of functional ingredients in wild macroalgae, there is less information available on optimizing these functional ingredients in aquaculture. Therefore, we performed controlled experiments to optimize the activity of antioxidants in Agarophyton vermiculophyllum (Ohmi) Gurgel, J.N.Norris et Fredericq comb. nov. (formerly Gracilaria vermiculophylla) grown in RAS with artificial seawater and commercial fertilizer. We show that the free radical scavenging activity could be increased by 13% via high salinity, and up to 34% by increasing the light intensity, but not daily light dose, for a period of 7 days. We also monitored growth rates and the maximum quantum yield of photosystem II (Fv/Fm) and show that the conditions for optimizing antioxidant activity are not optimal for growth or photosynthesis. We therefore suggest an optimization period of 4–7 days exposure to high light on a 6:18 hour light:dark cycle prior to harvesting in order to increase antioxidant activity.