Taking your example, when you look at the scale 7.0 is neutral. Anything less than that falls into the acid range. If the pH of a well buffered pond with baking soda is 8.2, you can see that the more 6.5-7.0 pH water you add, the more you are depleting the carbonates in the pond/tank.

Carbonates are expended in bio filtration and a minimum is needed to keep the pH stable. When the minimum are not met, you unfortunately see what happened in the OP's thread.

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