Potassium chloride has a solubility of 34.2 g / 100 mL of water @ 20 C, so our solution must be less concentrated than that. Let's choose an arbitrary concentration of approximately half that by dissolving 150 grams of KCl in enough water to make up a total volume of 1000 mL. Since the Molecular Mass of KCl is 74.55, that gives a molarity of 150 / 74.55 = 2.012M in our solution. Since the Atomic Weight of potassium is 39.0983, then each mL of our solution will contain 2.012 * 39.0983 = 78.67 mg of potassium. In order to raise the potassium level of 100 gallons of water by 10 PPM, we need to add 10 * 100 * 3.7854 = 3785.4 mg of potassium. So, it will take 3785.4 / 78.67 = 48.12 mL of our 150g / 1000 mL KCl solution to accomplish this. For your 127 gallons, you would need 48.12 * 127 / 100 = 61.11 mL of our solution. This is assuming your KCL is 100% pure; your MSDS indicates a slightly lower concentration of KCl in the substance you are using. Adjust the numbers above accordingly, if you wish.
