... soak it in water for a few days drain the water out and repeat until the ph stablizes...
Actually I was a victim of the slave trade in the swimming pool industry. When a pool is resurfaced, the surface coating is between 1/8 and 1/4 inch thick. Pools average between 12,000 and 18,000 gallons. Note the ratio of water to cement product. It takes about a year for the pH to stabilize

And that is with the addition of muriatic acid on a weekly basis. Muriatic acid is a 30% Ba of Hydrochloric Acid.
I am not saying to use muriatic acid in a fish tank (totally wrong thing to do). Vinegar is probably the wrong thing to do too. The best plan of attack is to try the salt thing and if there is continued pH fluctuation, then do the water changing thing.