When humans burn fossil fuels, greenhouse gasses including carbon dioxide (CO2), are released into the atmosphere. Nearly half of the added atmospheric CO2 is absorbed into the oceans, which cause the acidification of the ocean waters - termed ocean acidification. Concern that ocean acidification will severely impact the biodiversity of marine ecosystems has escalated. Average ocean pH has already declined by 0.1 units since pre-industrial times, and is predicted to fall another 0.3 - 0.4 units by 2100. Scientists were initially most concerned about the effects of ocean acidification on calcifying organisms. A range of impacts, including the dissolution of calcifying plankton, and a declining growth rate in reef-building corals have been documented. However, it was not until more recently that the impacts ocean acidification have on fish were assessed. Experiments have shown that the behavior of fish can be seriously affected by projected future CO2 concentrations. Documented impacts range from the iconic clownfish unable to process information from chemical cues important in the identification of their host sea anemones, to temperate sharks unable to properly track prey. Whether fish can acclimate to elevated CO2 over the longer term, and the consequences of altered behavior on the structure of fish communities remains unknown. However, evidence from natural volcanic CO2 seep reefs demonstrate the same behavioral abnormalities as shown with laboratory experiments. If fish behavior does not adapt to rising CO2 levels over coming generations, there could be serious consequences for the structure and function of future marine communities.