The cause of low pH in systems is due to a high level of carbon dioxide. That can be due to a number of causes. The most common is a high level of carbon dioxide in the room, due to people or pets breathing.
Sometimes, tanks have poor aeration, which allows carbon dioxide to build up in the water column. Better aeration, such as a skimmer or better circulation, can improve the pH by outgassing carbon dioxide. Aeration doesn't refer just to oxygen. It includes all the components in the air: nitrogen, oxygen, carbon dioxide, etc. We tend to focus on carbon dioxide and oxygen because they're the active components in our tanks, for the most part.

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