[Arthur]

Adding baking soda (sodium bi-carbonate) is not a solution.

A small amount of calcium chloride (CaCl) will stop the pH from going too high by precipitating calcium carbonates out of the water as the pH rises.

In a mature pond without adequate carbonates supply the pH tends to get down (and will eventually crash). It is not natural for a mature pond to develop a rising pH unless there is a problem with the source water OR things like lime rocks or fresh concrete.

It is not recommended to change the pH too fast as this is very damaging to the fish. The recommended method to keep the pH in a "normal" range is to put oyster shells or rice coral in the water flow. As bacteria "eat" the ammonia, they release CO2 which turns into carbonic acid, which in turn "eats" the carbonates in the oyster shells / rice coral to make bi-carbonates in solution.

The pH can get too high if their is a lack of GH (ie a lack of Ca or Mg in the water). Free Ca++ in the water will precipitate excess HCO2 to limit how high the pH can go.