CO2 can exist in water independently of dissolved oxygen. If the level gets too high, even with normal dissolved oxygen levels, illness will result.
Many atmospheric gases dissolve in water to some extent and carbon dioxide is one of the most important. It dissolves readily to form carbonic acid, giving a weakly acidic solution. This chemical reaction has important consequences, not only for the eventual chemical content of water, but also for other chemical and biological reactions that occur in the aquatic environment. The naturally acidic nature of rain allows it to bring other, less reactive, substances into solution. (The pH of naturally formed rainwater is about 5.6 but, as we shall see, local atmospheric and landscape variations can alter it significantly before it gets anywhere near our fish ponds.) The affinity between carbon dioxide and water has many far-reaching effects on water quality, playing a major role in plant and animal respiration and pH buffering.