Probably not unless you are into home canning tomatoes and maybe have some calcium chloride hanging around. First, however, a couple of questions: how do you know you need to lower the mash pH? Have you measured it previously? What kind of beer are you brewing and what is your water profile?
The pH of the water itself doesn't tell you much and in fact is of little importance. It is the interaction between the level of various brewing water ions, the water's residual alkalinity along with the type and ratios of malt types in the recipe that will determine the pH of the mash. If you don't already have one get a water report from your water supplier. Knowing what is in your water is the first step in making the proper adjustments for good brewing.