How much hydrochloric to decrease alkalinity?

Use the "acid demand" solutions in your test kit to determine how much acid is needed to bring the pH down to 7. There should be a chart in the test kit that shows how much acid to add depending on the gallonage of the pool. It may take several weeks of this to bring the total alkalinity into desired ranges. Lowering the pH more than that may strip copper off the heat exchanger in the heater. Typically you will knock 10 points or so off the TA reading every time you do this.