Coral-dwelling fish experience large daily fluctuations in oxygen, with hyperoxia occurring during the day and hypoxia building up at night. Nighttime hypoxia can be particularly severe between the branches of corals and intensifies in warmer temperatures due to the high respiring biomass of many shelter-seeking organisms. The damselfish, Dascyllus aruanus, is one of the species that seeks shelter within corals and must tolerate hypoxia on a nightly basis. Here we use a combination of field dissolved oxygen measurements, in situ underwater video recordings on the coral reef, and lab experiments to ask whether Dascyllus aruanus has evolved specific behavioural (e.g., changes in activity and sheltering) and physiological (e.g. changes in vision) adaptations to enhance performance under hypoxia. We compare these results to the nocturnal cardinalfish Ostorhinchus angustatus, which rarely experiences hypoxia, and predict it lacks hypoxia-related adaptations. Given the anticipated rise in severe hypoxia occurrences, understanding these adaptations is crucial for assessing the resilience of these species to a changing climate. By using a variety of methods, we aim to gain a new perspective on how oxygen shapes the behaviour and physiology of coral reef fish.