CO₂-induced aquatic acidification is predicted to affect fish neuronal GABA_A receptors leading to widespread behavioural alterations. However, the large variability in the magnitude and direction of behavioural responses suggests substantial species-specific CO₂ threshold differences, life history and parental acclimation effects, experimental artifacts, or a combination of these factors. As an established model organism, zebrafish (Danio rerio) can be reared under stable conditions for multiple generations, which may help control for some of the variability observed in wild-caught fishes. Here, we used two standardized tests to investigate the effect of 1-week acclimatization to four pCO₂ levels on zebrafish anxiety-like behaviour, exploratory behaviour, and locomotion. Fish acclimatized to 900 μatm CO₂ demonstrated increased anxiety-like behaviour compared to control fish (~480 μatm), however, the behaviour of fish exposed to 2200 μatm CO₂ was indistinguishable from that of controls. In addition, fish acclimatized to 4200 μatm CO₂ had decreased anxiety-like behaviour; i.e. the opposite response than the 900 μatm CO₂ treatment. On the other hand, exploratory behaviour did not differ among any of the pCO₂ exposures that were tested. Thus, zebrafish behavioural responses to elevated pCO₂ are not linear; with potential important implications for physiological, environmental, and aquatic acidification studies.