Total hardness therefore is the sum of the alkaline and non alkaline salts present in the water and these are the scale producing salts.
The addition of rain water, reverse osmosis or distilled water is the simplest way of diluting very hard water and so reducing general hardness.
i.e. Take the actual value of tap water hardness and subtract the value required (all values must be in same units) then devide this number by the value required, this will give the ratio of rain/distilled required.
example:-The total hardness of the tap water is 200ppm.The hardness required is 50ppm .200-50=150ppm 150 devided by 50 = 3 Therefore mix three times as much rain/distilled as tap water.
If living in a hard water area .Use of soft water systems which can be used exchange Calcium & Magnesium Ions but with this system the total disolved solids remain the same. The only water treatment system that will soften hard water with high Calcium and Magnesium is a cation resin that absorbs (actually removes,not replaces) those elements. There are also anion resins that removes carbonates and sulphates which also contribute to hardness. Hence the best propriety water softening products are that which discard (ion removers) not regenerated (ion exchange).
Aquarists work in systems based on the actual wieght ( not milli equiverlents ) and the German method is the most common because they wrote the first aquaist books.
Hence Calcium hardness was described as D.H. (Deutch Hardness) and Carbonate hardness as K.H. ( because the German for carbonate is Karbonate ) for example :- the German hardness d.H. (deucscher Harte ) became D.H. To this day British Fishkeepers talk of D.H. or G.H. ( general hardness ) which covers calcium & magnesium and K.H. ( carbonate hardness ).
The anion contributions to hardness are carbonates & bicarbonates. If acids are added to the water the H+ are taken out of solution, hence the expected p.H. drop does not occur. The americans call this buffering effect reserve alkalinity. This is important as sudden p.H. changes can stress fish. In the aquarium, the level of buffer is recomended to be not less than 2K.H. and not over 8 K.H. in fresh water.