Hardwater, Softwater decoded

What is 'hardwater'?

In the aquatic industry, the term "hard water" is generally used to refer to water with a high mineral content. This can mean that the water is either high in KH (high in carbonates) or high in GH (high in calcium/magnesium), or both. In layman's terms, it is common for "hard water" to be high in both GH and KH because hard water sources usually come from areas with limestone [CaCO3] - which contributes to both GH [Ca ions] and KH [CO3 ions].

However, hard water can be high in GH and low in KH and vice versa:

Minerals that can increase GH without increasing KH are minerals that contain calcium/magnesium but not carbonates.

Minerals that can raise KH without raising GH are minerals that contain carbonates but not calcium/magnesium.

Whether or not hard water is a problem depends on the exact GH and KH levels. Most animals and plants are more tolerant of high GH than high KH.

What is 'softwater'?

As with the term hard water, aquarists use the term soft water generically to refer to water with a low mineral content. However, we should actually define the variables by GH and KH.

Most "soft water" plant species are actually sensitive to high KH levels, not GH. It is more accurate to say that such plants prefer water with low alkalinity [low KH]. Most plants are tolerant over a wide range of GH, unlike KH.

0-2 dKH - you have very soft water, suitable for species such as Tonina, Syngonanthus, Blood vomit, Ammania.

3dKH - soft water, borderline for sensitive soft water species

4-7dKH - medium hard water/hard water, species such as Downoi do better in this range than in soft water. Soft water species such as Rotala macrandra can still do well in certain situations.

8-14dKH - firmly in hardwater territory. Soft water plants will grow very slowly or not at all.

14dKH+ - Very hard water, more species will grow slowly or lose colour. Choosing plants that thrive in hard water is a wise choice.

For detailed care guide on aquarium plants, click here.

To learn more about growing softwater plants, click here.

What about livestock?

In general, fish are also more tolerant of a wide range of GH than KH. Most commercially available fish species can live in a surprisingly wide range of GH & KH. However, certain specific species, and especially their eggs and fry, may require low GH along with low KH water to breed well or show their full colour, while other species prefer much harder water. You'd have to check for specific species. It is important to have about 4dGH if keeping shrimp & snails.

For details on water parameters for fish in planted tank, click here.

What is the best GH/KH for plants?

Generally speaking, most plants will thrive in softer, lower KH water. If you have a GH of 5 and a KH of 1, you have water suitable for growing 99% of commercially available aquatic plants. The few species that will grow more easily in higher GH/KH are Pogostemon helferi, Rotala 'Sunset' and some Valisneria species.

In terms of growing plants, between 0-2 dKH you can keep sensitive soft water species. Between 2 - 6 dKH you can keep 97% of all commercial aquatic plants in optimum condition. Between 6-12 dKH you can probably grow 90% of species well, but some will be sub-optimal. Above 18 dKH or so, there are more problems with plant growth - at this level, hardy plants such as Java fern, anubias, vals, certain swords and crypts will still grow well, but many other species will stunt.

Below is a picture of a 2Hr tank with limestone, where the average is around 10 dKH/GH.

Is there a need to buffer water to 3dKH to prevent pH swings?

There is absolutely no need to do this unless you are specifically doing this to accommodate animals that favour a certain pH/KH. Contrary to popular belief - low KH doesn't cause pH crashes without additional triggers. This is a remnant of old aquarium dogma.

Similarly nitrifiying bacteria can fully function in low pH ranges.(read this link) For many CO2-injected, planted tanks, the mean pH can be consistently close to 6 or below. Virtually any aquasoil tank in soft water countries, of which there are thousands of examples (buffering KH to near 0 levels), will function well at such levels.

In our Dutch tanks, where we keep sensitive Eriocaulon species, the GH is around 3 and the KH is below 1 (usually around 0.5).