An acceptable pH for a reef tank is between 7.8 and 8.5. However, I have found my SPS corals do better when pH is in the 8.1 to 8.5 range.
There are several reasons why pH might be chronically low (7.7 to 7.9) in an aquarium. The tank could be in a room with poor air ventilation, causing C02 in the space to build up and lower the pH (C02 in the room is dissolved into the tank’s water).
Carbon dioxide impacts pH because it rapidly turns into carbonic acid when it enters the water. Acids lower pH, so more carbon dioxide means more carbonic acid, which means lower pH1.
A calcium reactor can also lower a tank’s pH since C02 is injected into the reactor. The C02 helps to dissolve the calcium-based media inside the reactor. This calcium-rich liquid or effluent is then dripped back into the tank.
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