Thank you for providing the extra information specifically for fish tanks.
There is some standard information that has been propagated in the aquarium industry for years. Since many of the fish that people are trying to keep are from the Amazon and since the pH of the Amazon water is so low. Many people attempt to lower the pH in their fish tank with peat to duplicate the water conditions.
There is a "new" school of thought that a higher pH with sufficient supplies of calcium and magnesium in the water (sound familiar) would be optimal but old habits die hard.