| The pH of 6.4 is not the problem. The problem is that it indicates you have no buffering capacity and that the pH will decline as the acids produced by decay and nitrification accumulate in the system. Since its a logarithmic scale, the lower the pH becomes, the faster it will drop. A pH "crash", as they say.
Sometime when there's a moment, you should spot check your tap water pH after it has been aerated (in a separate container and before exposure to the oyster shell). Some water supplies have excess carbon dioxide (a weak acid). After aeration, carbon dioxide will come into equilibrium with the atmosphere and you could find that the aerated pH is above pH 7.
Having oyster shell in the system is a good thing. But, it dissolves slowly and there is an outside chance that you are less reliant on the oyster shell than you think.
-steve |