The carbonate is acting as a buffer... you want to add acid to the water to decrease the pH, not another buffer. Got a friend with an established pond or aquarium? Borrowing a few buckets of water from them is often the fastest way to get a new pond or aquarium to "settle in", and then you can adjust from there. http://www.wqa.org/glossary.cfm?gl=626 Carbonates can buffer soils in the same way as water. http://www.cals.cornell.edu/dept/flori/growon/field.html#beginning