Some of the most compelling evidence in support of the oxidative stress theory of coral bleaching, comes from three studies that expose corals, cultures of their algal endosymbiont, or the coral model Exaiptasia diaphana to exogenous antioxidants during thermal stress. Here, we replicate these previous experiments using E. diaphana with the addition of mannitol, ascorbate plus catalase, or catechin under ambient or elevated temperatures along with an antioxidant-free control. We tracked physiological parameters and quantified reactive oxygen species production over two weeks. We found that in elevated temperature conditions, compared to the no antioxidant control, anemones treated with mannitol or catechin had significantly higher photosynthetic efficiencies. The combination of ascorbate and catalase significantly reduced physiological performance under elevated temperatures and caused mortality in the anemones. This study is now one of only four to have evaluated the use of antioxidants as a prevention of bleaching and will be used to drive assisted evolution strategies, such as the application of probiotics, to mitigate the effects of climate change.