Why does adding CO2 to home aquaria benefit animals, but in the ocean, adding CO2 leads to harmful acidification?
Freshwater fish and plants tend to be more tolerant of lower pH and wider pH changes overall because fresh water contains low alkalinity, which means that the water chemistry does not minimize pH changes (i.e., it does not have the “buffering capacity”) the way that seawater chemistry does. The natural variability of pH in lakes and rivers is also higher than in the ocean. Freshwater organisms have evolved special mechanisms that allow them to thrive in these more acidic and variable conditions; for example, freshwater plants may benefit from higher CO2. In saltwater aquaria, corals and fish require a more narrowly balanced pH and owners often add carbonate “hardeners” to increase the water’s alkalinity and maintain the pH between 8.0 and 8.4. Devices called “calcium reactors” bubble CO2 gas through crushed calcium carbonate (usually crushed coral), which releases calcium and carbonate ions into the salt water, providing the high-alkalinity, calcium-rich waters that aquarium corals and
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