Just to throw in my thoughts…
Phosphoric acid should be safe for this, but if I were doing that I would actually put the water in a container outside, with some plants (Water hyacinth would work very well here), and leave it alone while testing the pH at regular intervals.
Plants will also remove chelated metals from water (especially iron and magnesium), and reducing the pH will improve the ability of the plants to absorb them. The only thing that I would say (especially if somebody reads this thread and doesn’t have much knowledge of chemistry) is that one either needs to use the equation to determine the change in pH (not too hard if you have a digital probe)and add the amount called for via the calculations or add a small amount at a time and check the pH intermittently.