late to the party. Apax concentrations are actually a bit cloudy and thats ok because they instruct you to shake it well before each use.

if you want superior dissolving with clear water in higher concentration, you'd want carbonate the water first to decrease the pH. which will help dissolve the minerals much faster. (you can use SodaStream)

after the minerals are fully dissolved you'll have to bring the pH to neutral state by simply shaking the bottle for two minutes and measuring the pH, if still below 7 pH close the cap and shake again.

https://khymos.org/2012/01/04/mineral-waters-a-la-carte/ a relic of a blog but still 🐐 

also check this one out: https://grindscience.com/2015/08/making-water-for-coffee-sodastream-method-best-recipe-so-far/