HARD WATER, SOFT WATER and plant growth
WHAT IS 'HARD WATER'?
Most aquarists use the term "hard water" to refer to water that has a high mineral content - this can mean either that the water has a high KH, or a high GH (high calcium/magnesium content) or both. In layman's terms, "hard water" usually has both a high GH and a high KH because hard water sources usually come from limestone areas [CaCO3].
In fact, hard water can be high in GH and low in KH or vice versa: minerals that can increase GH without increasing KH are minerals containing calcium / magnesium without containing carbonates. Magnesium sulphate [MgSO4] and calcium sulphate [CaSO4] can both increase GH without increasing KH. Minerals that can increase KH without increasing GH are minerals that contain carbonates but no calcium / magnesium: potassium carbonate [KCO3] and sodium bicarbonate [NaHCO3] both increase KH but leave GH unaltered.
Whether or not hard water is problematic depends on the exact GH and KH values. Most animals and plants are more forgiving of high GH than of high KH.
WHAT IS 'SOFT WATER'?
Just like with the term hard water, aquarists use the term soft water to refer to water with a low mineral content. We really need to define the GH and KH variables precisely.
Most "soft water" plant species are actually sensitive to high KH values but not to GH. Therefore, it is more accurate to say that such plants prefer water of low alkalinity [low KH]. Most plants tolerate a wide range of GH as opposed to KH.
0-2 dKH - you have very soft water, suitable for species such as Tonina, Syngonanthus, Ammania.
3dKH - soft water, limit for sensitive soft water species
4-7dKH - medium hard water / hard water, species like Downoi do better in this range than in soft water. Soft water species like Rotala macrandra can still do well.
8-14dKH - firmly in the hard water area. Soft water plants will grow with difficulty or not grow at all.
14dKH+ - Very hard water, more plant species will grow slowly or lose their colour. It is wise to choose species that will do well in hard water.
WHAT IS THE BEST GH / KH FOR PLANTS?
In general, most plants do better in softer, low KH water. If you have a GH of 5-6 and a KH of 0-1, your water is suitable for growing 99% of commercially available plant species with ease. The few species that grow more easily in high GH / KH values are Pogostemon helferi, Rotala 'Sunset' and some species of Valisneria.
When it comes to growing plants, you can keep sensitive, soft water species between 0 and 2 dKH. You can keep 97% of all commercial aquatic plants in optimal condition between 2 and 6 dKH. You can probably grow 90% of species well between 6 and 12 dKH but some may not do their best. You'll encounter more problems with plant growth above 18 dKH. Hardy plants like Java varen, Anubias, Val and certain Swords and Crypts will still grow well, but others will struggle.