Sodium bicarbonate is your answer.

Use baking soda (sodium bicarbonate). It raises total alkalinity without a big jump in pH.

Soda ash (sodium carbonate) raises both alkalinity and pH because its pH is around 11. Baking soda sits around pH 8, so when you add it to pool water (typically 7.2–7.6), the pH barely budges. It’s the standard trick for fixing alkalinity without overshooting pH.

Add it slowly — about 1.5 lbs per 10,000 gallons to raise alkalinity by 10 ppm — then test. Wait a few hours between additions. If pH does creep up, a bit of aeration (running fountains or returns) can bring it back down.

Just don’t overdo it — high alkalinity can cloud your water and lock pH in place.