Water testing series

Part 6 - General Hardness - GH

Do you have hard water? or soft water? You need to test your GH to find out!

General hardness is the measurement of salts and ions dissolved in water. In freshwater fishkeeping, we’re mainly concerned about Calcium [Ca] & Magnesium [Mg].  Both of these elements are likely to be present in tap water, but at what quantity, can depend greatly from water source to water source. When the term ‘hard water’ is mentioned, it is referring to water with high levels of these two elements (not necessarily both, it can be just be one of them, or more of one than the other). ‘Soft water’ on the other hand, is water with low quantities of these two elements.

As mentioned above, there are several different measurements of general hardness, which may also be called permanent hardness. The easiest way to avoid confusion is to stick to one measurement, but alas, this isn’t always possible. Some websites, books, manufactures, water suppliers, and hobbyists may use different ones to each other.

The degrees and PPM & mg/l

German, French and English (known as Clarke) degrees of hardness are different to each other. German degrees of hardness tend to be the most popular in the aquarium hobby.

Parts Per Million (PPM) and milligrams per litre (mg/l) are also a popular measurement of hardness, but just using ppm or mg/l at the end of a value doesn’t actually mean anything unless you clarify what they are a measurement of. The most popular compound used as a measurement is Calcium Carbonate [CaCO3]. So, for example, you might say “the general hardness of this water sample is equivalent to 17.9 mg/l CaCO3”, although this doesn’t necessarily mean there is this much Calcium present, just that it’s the equivalent hardness if there was this amount of calcium [Ca] present.


There are several online conversion calculators to convert one measurement to another. This can save a lot of headache trying to understand the differences. Generally speaking, most test kits and informational sites will either use the German scale for hardness or offer several, but just make sure you find out which measurement they use!

To understand what an ion is, you must first understand the makeup of an atom. In a simplified explanation, an atom is made up of a nucleus (centre) with ring(s) of electron(s) spinning around the nucleus. The nucleus is made of protons & neutrons. As the name might suggest, a proton has a + charge, a neutron has no charge and an electron has a – charge. In (most) element’s base form they will have an equal number of protons to electrons, and so will be neutrally charged.

Depending on the element, as well as where it is found, there is a possibility for the atom to be found as an ion. An atom is in an ionic state when it has either lost or gained electron(s). If an atom has gained an electron, it will have an overall negative charge and is called an Anion. If an atom has lost an electron, it will have an overall positive charge and is called a Cation. This is why metal cations (+ charged ions) such as sodium, potassium, magnesium & calcium can form ionic bonds with non-metals such as chloride. The best example of this is NaCl, or as it is most commonly known, table salt. All ionic bonds (a cation [+ion] bonded to an anion [-ion]) are types of ‘salt’ (this is why fresh aquarium salt blends, marine salt blends, table salt & even Epsom salts, are all different).

Level of General hardness

General hardness can be measured in a few different ways. The two most popular scales of measurement are Degrees of General Hardness and mg/l (milligram per litre) of calcium carbonate (CaCO3). Both measurements are often used, but for the sake of simplicity, we’ll be using German Degrees of Hardness (dGH).

Below is a rough guide of what different levels of GH are best suited for. Please do your own research before purchasing particular species and variants of fish and invertebrates!

0 dGH

No general hardness at all. Not recommended for keeping any fish or inverts.

3 - 6 dGH

Very soft water. Some species of fish may prefer this level of hardness such as discus and other South American Cichlids. Shrimp species such as caridina shrimp will also prefer this level of GH (most types).

6 - 12 dGH

Moderate hardness. The majority of aquatic plants will thrive in this hardness. Ideal for most soft and moderate hardness fish. Ideal for ponds. Ideal for species of shrimp such as neocaridina shrimp (most types).

12 - 18 dGH

Moderatly high GH. Ideal for Rift Valley Cichlids (Lake Malawi). Ideal for brackish species.

18+ dGH

High KH. Not recommended to keep freshwater fish.

Why is the hardness of water important?

The hardness of water is incredibly important when it comes to keeping fish, invertebrates and plants alive in an aquarium, mainly because each species (sometimes even down to the variant) will have a preferred level of hardness. There are two main reasons why each species will have their own preferred range of GH  


To understand why different species of fish and inverts need particular levels of general hardness, you need to understand Osmoregulation. Osmoregulation is the process by which a fish balances out the water and salts within its body in relation to the salts in the water outside its body. Osmoregulation is essentially the way fish can control osmosis within their body.

Osmosis is where water will move through a semi-permeable membrane from a lower solute concentration (salt for example) to higher solute concentration. In fish, the semi-permeable membranes are cells walls and tissues (gills for example), and the solute are salts and ions. A real-world example of osmosis can be found when curing meat with salt, as this will pull water from the lower concentration side (the meat) to the higher concentration (the salt blend) leading to a dryer meat.

As each species of fish have evolved in water bodies with particular levels of hardness, salt and other ions, they have adapted to take in and get rid of the right ratio in order to maintain the right level of water within their bodies. Moving fish from their preferred or adapted level of general hardness to a different level can lead to osmotic shock and death. This can be somewhat mitigated by acclimatising livestock to different levels, but major difference in hardness should be avoided if possible.

Essential minerals for life.

Providing the right levels of magnesium and calcium is also important for the fish to grow healthy bones & tissues, support a healthy immune system, aid in digestion and all other metabolic reactions. They are an essential building block. Unlike terrestrial animals that may be able to get these minerals from food, fish can absorb these elements through the water. It’s also worth noting that aquatic plants will also use these elements to facilitate growth.

In some of the natural habitats of tropical fish, a raining season is common. Raining seasons and increased rainfall will usually lower the GH of water by diluting the water source. Many fish have adapted to breed during the Raining season and can be triggered to enter breeding condition when the GH of water drops. This doesn’t necessarily work for all tropical fish, and there may be other factors associated such as a change in temperature and pH.

Can I change the GH of my water?

It is possible to change the GH of water both before it goes into your aquarium and after. Treating water before it goes into your aquarium is an easier and safer method to avoid stressing livestock. GH will naturally deplete over time, in an aquarium due to plants, fish and inverts taking the calcium and magnesium out of the water (and through these minerals precipitating).

↑ ↓ Continue using Tap water for water changes

In areas with moderate and high tap water hardness, it’s possible to maintain and raise GH by continuing to perform regular water changes. If done frequently, GH will stay fairly stable. Always make sure that you condition tap water to remove toxic compounds such as chlorine!

Reverse Osmosis (RO).

Using RO water can help dilute GH and therefore lower hardness in an aquarium. Be careful not to overuse RO on its own as the hardness could become too low (even for soft water species). Using only RO water can be done, but remineralisers should be used.

Use GH Boosters

In soft water (such as RO), a remineraliser can be used to add the essential elements back into water. There are several options, but it is worth researching which will be better for your fish, inverts and plants.

For example, some remineralisers will add not only Calcium & Magnesium, but also other vital nutrients such as Sodium (Na) and Potassium (K). This can be beneficial for some species of fish (livebearers such as mollies) that prefer a certain level of sodium, but it can also be detrimental to soft water species, which are not tolerant to excessive sodium salts (neon tetras). Alternative remineralisers are available (Seachem Equilibrium) to suit these soft water species, as well as plants, and will either contain lower sodium content or none at all.

Use Coral Stone & Aragonite

Stones that contain Calcium Carbonate (CaCO3) will naturally dissolve over time and release a source of Calcium. (This will also raise KH, and therefore may alter pH). Coral stone can either just be added to the aquarium, or crushed coral stone can be added to filters.