Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If the KH is really low to begin with (under 3 German degrees of hardness) then adding something like peat, driftwood of any sort, or other organic matter can indeed crash the pH.
I would remove the wood and soak it in several water changes, boil it if possible. You might get an idea if this really was the problem by checking the GH, KH and pH of the water you are soaking it in.
Then check the KH of the tank. If the Manzanita is continuing to reduce the KH in the soaking water, even after several water changes (like a week with twice daily water changes) then you will have to monitor the tank very carefully, adding baking soda for a quick fix, but adding the coral sand to the filter would be a better solution. Slow release of the minerals, and the coral sand dissolves faster when the pH drops, then slower as the pH rises, which is exactly what you want.
Other materials that do the same thing are limestone sand or gravel, and oyster shell grit. This is sold for caged birds like Budgies. I use a blend or one bag of each coral sand and oyster shell grit in my hard water tanks.
I have several substrates that do the same thing: Remove all the KH, that allows the pH to hit the bottom of the test. The fish are fine as long as they are acclimated, and the change takes place slowly. I think it was the sudden onset, and the removal of the minerals from the water not the actual endpoint of the pH that caused the problems. I have soft water fish in these tanks (many species- Tetras, Barbs, Loaches, Cichlids, Cats and others).
In my hard water tanks (Live bearers, Rainbows, Lake Tanganyikan fish) I do not allow that to happen. I use GH booster and baking soda, but also coral sand and oyster shell grit to keep the minerals high for these fish.