Extreme habitats challenge animals with highly adverse conditions, like extreme temperatures or toxic substances. In this paper, we report of a fish (Poecilia mexicana) inhabiting a limestone cave in Mexico. Several springs inside the cave are rich in toxic H2S. We demonstrate that a behavioural adaptation, aquatic surface respiration (ASR), allows for the survival of P. mexicana in this extreme, sulphidic habitat. Without the possibility to perform ASR, the survival rate of P. mexicana was low even at comparatively low H2S concentrations. Furthermore, we show that food limitation affects the survival of P. mexicana pointing to energetically costly physiological adaptations to detoxify H2S.