I have read that baking soda will bring it down, why is so high? Do you have concrete in the pond, shells, wood?
from a fish forum
"Should you mess with the pH? In my opinion you are probably just as well off to leave it alone, even if it is high. You should check the buffering, AKA alkalinity, or sometimes called kh. It is probably high, but you should check it. If your buffering is between 100 and 400 you should not try to mess with the pH. If it is higher you could think about adding some acid, but watch the buffering closely, and don't pay too much attention to the pH. Usually people get into trouble by only watching the pH. It won't change much as you add acid, then all of a sudden after you have broken the buffer, the pH will crash and kill your fish. Pay attention to the kh!!!"
Baking soda is really the only product that should ever be used to drop the pH slowly and stabilize it. But like others have said, it is usually safer to just leave it alone, if it is stable.
A lot of times you'll get a high pH right out of the faucet from lack of CO2. Aerating it will drive more CO2 into the water, bringing the pH down.
Because of its chemical makeup, Baking Soda has unique capabilities as a Ph balancer or buffer. Buffering is the maintenance of a stable pH balance, or acid-alkali balance. As a buffer, Baking Soda tends to cause acid solutions to become more alkali and to cause alkali solutions to become more acid, bringing both solutions to a stable pH around 8.1 (slightly basic) on the pH scale. A buffer also resists pH change in a solution, in this case maintaining a pH of 8.1." The nature of baking soda is that it will give up H+ ions making more acids,
but only if the pH is above the buffer point. When it does this, it leaves behind carbonate ions, which could precipitate as calcium carbonate, (limestone), of calcium magnesium carbonate, (dolomite), if there is enough
carbonate and it is not being pushed back to bicarbonate. "
