The online publication of the Minnesota Aquarium Society Aquazine Minnesota Aquarium Society since 1931 Hardness Provided by the Minnesota Aquarium Society Conversion Calculator Hardness Enter value to be converted (use mouse number buttons or enter in input field at left) and press one of the unit of measure buttons at the top. Parts per Million(mg/L) DH Milligrams/gallon Other Conversion Calculators Linear Temperature Volume