Just to clarify about hardness. Hardness is not the same as TDS.
GH or hardness is the amount of divalent metal ions in the water. In practice, this means mainly calcium, some magnesium and trace amounts of other metals.
Hardness does not measure monovalent metals such as sodium and potassium, or trivalent metals such as aluminium. This is why some water softeners use salt to exchange the hardness minerals for sodium - sodium isn't part of hardness.
Hardness, GH, does not measure things like ammonia, nitrite or nitrate. Or carbonate. Or organic chemicals such as tannins and fish hormones.
Total dissolved solids (TDS) measures just about everything in the water. TDS includes everything in GH, plus KH, plus things like sodium and aluminium, plus organics etc.