I bought the cheapest kind, Poland Spring 40% proof. I should clarify I am not sure how the Alk was measured. It was part of some dip strips I had. I never paid much attention to it; just thought I would include it. My Cal was always between 400ppm and 430ppm during monthly water changes. I have been dosing Reef Plus Concentrated vitamins & amino acid supplement along with Kent Marine Strontium & Molybdenum weekly. Approximately, 5ml each.