Can baking soda be used instead of soda ash in a salt water a pool?

Yep! Baking Soda is Sodium Bicarbonate. It's the same stuff as "Alkalinity UP" but much cheaper. If you get the bags of it at a warehouse store, instructions for use in a pool will be on the back. New pool owner here. I had the same question, and given that the pool is a captive body of water, came to the conclusion that one simply cannot substitute Baking Soda (Sodium BiCarbonate) for a PH increaser (i.e. Sodium Carbonate) without fully understanding what affect the chemical structure difference between the two will have on the water. I enclose here some info on the chemical makeup of the two: Sodium carbonate, chemical formula Na2CO3 is less acidic, than sodium bicarbonate (baking soda), whose chemical formula is NaHCO3.