you can water with an acidic solution. You can dilute sulfuric acid, purchase "pH Down" which is used in hydroponics (just concentrated acids), or you could use vinegar.
Get some of the pH testing drops so you can test the pH of the water before using.
That's what I'd do in your situation.
If the plants were in the ground, you could still water with acidic water but you could also use elemental sulfur around the plant/tilled into the top of the soil. It slowly brings down the soil pH but may not keep it there without repeated applications.