pH, KH, GH, TDS - Introduction to basic water parameters

Basic water parameters and how they affect an aquarium

pH

pH is a measure of how acidic or alkaline the water is. A pH reading of 7 is neutral, while numbers below 7 are acidic and numbers above 7 are alkaline.

What is pH actually

pH stands for "Power of hydrogen". A technical description of pH is that it is the ratio of positive electrical charges from ionized hydrogen cations (H+) to the negative electrical charges from hydroxyl anions (OH-). The pH scale is logarithmic, every 1 pH represents a 10 times difference in acidity from the previous number, so a pH of 5 is 10 times more acidic than a pH of 6.

How much does pH actually matter in an aquarium?

As a general rule, being on the generally correct side of the scale give the best outcomes for both fish and plants. This means that soft water plants/fishes should be kept in neutral to acidic conditions while hard water fish/plants do better on the neutral to alkaline side of the scale - the exact degree of acidity matters less except for particularly sensitive species that only do well in a narrow range. 

Most commonly available fishes have been bred in aquarium conditions for so long they are flexible within a wide range of pH. You will see farmed Cardinal tetras, which come from more acidic water in the wild (pH 5+) regularly being kept in alkaline water tanks (pH 7-8+) without much issues. Wild caught fish and fish that has not yet undergone many generations of artificial breeding can still require very specific conditions in comparison.

Ideal conditions often trigger better fish colouration and spawning behaviour. It is therefore important for breeders and folks that participate in fish competitions.

Many fish species show better coloration and spawning behaviours when kept in conditions that match where they come from. Hyphessobrycon myrmex are native to the Rio Orinoco basin from from South America and show better coloration when kept in acidic waters.

Is a stable pH important?

pH changes in nature are common, due to rain fall, floods that stir up sediment, and changes in carbon dioxide levels.

Dissolved Carbon dioxide [CO2] produces carbonate acid and causes the acidity of the water to fall [Similar to how carbonated fizzy drinks are produced]. Carbon dioxide in nature is produced by under ground aquifers and decomposition of organic material. CO2 causes the pH to fall as it builds up over night in rivers and lakes. When the sun rises, aquatic plants absorb the CO2 when they photosynthesize, reversing the effect that dissolved CO2 has on pH, causing the pH to rise. The effect of this is that many natural water bodies can have significant pH swings from sunrise to noon time.

Giant springs, Montana. 25ppm of CO2 measured. Photo credits: Tom barr.

In aquariums where there is no CO2 injection/generation, such as fish-only aquariums without plants, non-intentional changes in pH can signal deteriorating biological conditions which can be major red flags. A mass fish die off for example, will drive up carbon dioxide levels through decomposition, while depleting oxygen levels at the same time. This results in the pH dropping precipitously. The depletion of oxygen and deterioration of water quality is what affects aquarium inhabitants. The change in pH is merely a symptom of underlying problems - and rarely the cause of problems by itself. Unfortunately, this has given rise to the myth that pH swings are deadly by themselves, a statement that clearly mistakes symptoms for causation.

Large pH swings can also be caused by large changes in KH (carbonate hardness). Significant changes in carbonate hardness will affect fish osmoregulation. KH changes will always cause changes in pH, however, it is the underlying KH swing that affects fish.

In a planted aquariums with CO2 injection, it is normal for the pH to drop significantly as the water gets saturated with CO2 levels. In CO2 injected tanks pH can rise and fall over 1 to 1.4 degrees over a 24 hour window. This pH change does not change the underlying KH and this kind of pH swing does not have any significant impact on livestock.

CO2 injected planted aquariums can get away with large pH swings and low pH ranges that are usually outlier scenarios to regular fish-only aquariums. Even more sensitive organisms such as CRS shrimp reproduce readily in CO2 injected aquariums that see such large pH fluctuations on a daily basis.

Do aquatic plants care about pH at all?

Aquarium with a mix of exotic species such as Eriocaulon quingulare, Centrolepis drummondiana and Rotala florida. pH is around 5.2 for this aquarium.

Aquarium plants overwhelmingly prefer an acidic substrate, low alkalinity (low KH) and high CO2 levels (which also causes the pH to fall). The combination of the above always results in lower pH environments. This does not mean that one should target the end result (a low pH) but rather, it is a common outcome of providing the plants the variables they prefer (such as elevated CO2 levels and low KH).

This does not mean that alkaline water aquariums are doomed to fail with aquatic plants. Many plant species tolerate alkaline water well, such as Cryptocoryne species and hardy plants such as Anubias and Java fern. Vallisneria species are one of the few that actually prefer more alkaline environments, and grow well in more alkaline tanks.

image: CoolKoon

pH can affect nutrient availability. The chart above shows the effect of soil pH on nutrient availability. Foliage feeding studies show similar outcomes. It is debatable to what extent does this play a role in planted aquariums. Arguably, lower nutrient availability can be compensated by having a higher concentration of said nutrients. In aquariums, the main concern seems to be on micro nutrient availability - common micro-nutrient chelates (such as EDTA) breakdown more quickly in alkaline water, reducing availability to plants.

There is also some evidence that planted tanks without CO2 injection do better in acidic side of the pH scale. It is still in debate whether this is due to easier uptake of CO2 in low alkalinity (low KH) conditions or simply that low tech tanks that have meaningful CO2 generation see lower pH ranges due to increased CO2 saturation.

GH (General Hardness) & KH (Carbonate Hardness)

GH principally measures the amount of calcium and magnesium in water. Technically, it measures all divalent (2+) cations, however, calcium and magnesium are by far the most common divalent cations present in tap water. Calcium and magnesium are used as nutrients for aquatic plants while shelled organisms such as shrimps and snails require calcium for formation of their shells.

KH measures the amount of carbonate (CO3) and bicarbonate (HCO3) anions present. The higher the carbonate hardness, the higher the pH will be in the absence of other chemicals in the water, and the more resistant the water will be to downward fluctuations when an acid is added. Carbonate hardness is usually measured and quoted in degrees (dKH). Carbonate hardness affects fish osmoregulation and large fluctuations in this parameter is stressful to fish.

Large changes in alkalinity affects both fish and shrimp and should be avoided. Large changes in alkalinity also causes large shifts in pH - and this is probably where the idea that "pH swings are dangerous" come from.

What is hard water, is it high GH or high KH or both?

The term hard water refers to water with high mineral content. In nature, this typically means the water has passed through limestone, and would raise both GH and KH levels. In aquarium culture, this term is used loosely to refer to either high GH or KH or both. For precise discussion it is better to separate the two as they can be manipulated independently.

These are common breakpoints for soft vs hard water classification is as follows:

Organisms that require "hard water" may require high GH or KH or both. To tune parameters accurately, we need to know whether they are sensitive to one or the other or both. For example, Crystal red shrimp prefer low KH (0-1 dKH) combined with some GH (3-6 dGH), whereas lake Malawi cichlids prefers higher values for both GH and KH (10-20 dGH/dKH). Many livebearer prefer harder water, while most amazonian fish species prefer soft water.

Hyphessobrycon wadai from South america. Its natural environment is typical of many amazonian habitats - low KH (less than 2 dKH) with low pH. While it can survive well in hard water, it shows off better colouration in soft water.

Soft water plants are sensitive only to KH and can tolerate a much wider range for GH. For example, a tank with 7 dGH and 1 dKH can still grow any sensitive soft water plant. However, the reverse is not true - soft water plants will not do well in a tank with 1 dGH and 7 dKH. For folks looking to grow sensitive soft water plant species, having low KH (2dKH and below) is the critical factor. 

Many difficult aquatic plants such as Eriocaulon quingangulare and Centrolepis drummondiana require low KH to grow well long term. The tank parameters above are 5 dGH and 0.5 dKH.

How are GH and KH connected?

Limestone, which is extremely common in nature, often affects both parameters together, so it usual to observe that if a water source has a high GH, that it will also usually have a high KH. 

Seiryu rock (show above) is a rock that is popularly used in aquascaping. However, as it raises KH levels in an aquarium (ranging from 4 dKH up to 10+ dKH depending on the type and amount of rocks used), it is unsuitable for tanks that aim to grow soft water plants.

On a technical level, GH and KH can be entirely separate as they are affected by different ions. This is especially so in aquariums where we can add chemicals that raise one without affecting the other. Adding calcium sulphate into the water for example, raises GH (it contains calcium) without changing KH (it has no carbonates). Adding sodium bicarbonate into water raises KH (it contains carbonates) while it does not affect GH (contains no calcium or magnesium). It is perfectly fine to have an aquarium that has one value higher or lower than the other, depending on what your tank inhabitants require.

Here are some examples of common compounds and whether they contribute to GH or KH.

Seiryu rock, which is commonly used in aquascaping is a form of weathered limestone. It raises the GH, KH and pH of the time over time as it slowly dissolves. 

How is pH and KH connected?

Any change in KH will always result in a change in pH, but not all changes in pH change KH. Aquasoils and peat substrates absorb KH, which causes pH to fall at the same time. Adding baking soda to the tank raises KH, which will also cause pH to rise.

While large KH swings can affect fish, pH swings by itself do not. A large KH swing will always cause a pH swing. 

However, there are many weak acids, such as dissolved carbon dioxide and humic acids, that we can introduce into the aquarium that will change the pH without significantly changing the KH. CO2 injection for example, drops pH significantly without reducing KH.

How is GH and pH connected?

They are not connected at all. However, some minerals such as limestone, change both GH and KH at the same time. As KH changes always change pH, this means that limestone raises GH, KH and pH all at the same time.

There are many other elements that affect GH or pH alone, without changing the other. CO2 injection changes pH but has no impact on GH. Similarly, plants uptake calcium and magnesium over time for growth, this causes GH to fall over time, but this has no impact on pH either. Peat and other botanicals affect pH, but have no impact on GH.

Do I need KH to buffer my pH? Do I need to "stabilize my pH"?

Carbonates (which up KH) are consumed by ammonia oxidizing bacteria when they process ammonia to nitrates. In alkaline environments (pH 7+), ammonia oxidizing bacteria (AOB) are very efficient and may play the dominant role where ammonia oxidation is concerned. However, in acidic environments where carbonates are lacking, ammonia oxidizing archaea (AOA) play a more dominant role instead. AOB has also shown a greater affinity for high ammonia environments while AOA is more dominant in low ammonia environments.*

This means that unless you are running a high yield fish farm or fish heavy setup with heavy feeding where AOB is used as the primary ammonia oxidizer and are consuming carbonates at a tremendous rate, there is absolutely no need to maintain an elevated KH level in your tank. In more acidic, low KH tanks, microbes that are suited to acidic systems will develop over time and become the dominant species instead.

*Aquarium Nitrification Revisited: Thaumarchaeota Are the Dominant Ammonia Oxidizers in Freshwater Aquarium Biofilters by Laura A. Sauder, Katja Engel, Jennifer C. Stearns
, Andre P. Masella, Richard Pawliszyn, Josh D. Neufeld

"I don't see any compelling reason to increase KH for plant centric tanks with livestock not requiring alkaline conditions, especially if someone is using aquasoil. I think there are two primary reasons why many people advocate for high KH; One is this so called fascination about select groups of nitrifying bacteria, which don't exist in any meaningful way in most healthy tanks including tanks with higher KH (in plant centric tanks with livestock not picky about KH). As far as I know, healthy tanks generally don't produce enough ammonia to sustain a huge population of these bacteria because their enzymes have low affinity for ammonia, they need higher ammonia concentration to thrive. That's why these are primarily found in decent numbers in waste water treatment facilities and contaminated water bodies. There are several scientific studies on healthy freshwater systems including aquariums which show that these popular nitrifying bacteria (in the hobby) are not the primary nitrifying microbes (particularly for the first and the most difficult reaction that is conversion of ammonia to nitrite). I have done two separate DNA analysis from multiple tanks and I see the same thing. Not even a single tank showed any meaningful presence of these legendary nitrifying bacteria. Another reason for many people to stay obsessed with this higher KH theory is that they find it difficult to comprehend that KH is not the only factor that can maintain a stable pH (buffering capacity). Tanks with aquasoil also maintain a stable but low pH for decent amount of time. There are other factors as well such as application of phosphate as fertilizers for plants. Phosphate is a common buffer and is one of the primary buffers used in biochemical research. When applied on a regular basis as fertilizer in tanks with regular maintenance (weekly water change, substrate cleaning etc.), it can also maintain a stable pH in tanks with inert substrates with almost 0 dKH as well. This would mainly apply to people who dose ei. The point is, it is certainly possible to maintain low and stable pH in planted tanks which should be the main objective rather than getting obsessed about achieving some magical pH by increasing KH. I have not even mentioned about the effects on CO2 uptake by plants in higher KH water and potential negative effects on Fe and other metal uptake mechanism. There is a reason why most people struggle to grow plants in relatively higher KH water."

With the exception of aquariums using Seiryu rock, every aquarium showcased on this page runs near 0 KH, with 1+ degree pH swings due to CO2 daily.

Total dissolved solids (TDS)

TDS stands for Total Dissolved Solids and measures the total amount of dissolved substances, both organic and inorganic in the water column. This includes a whole range of dissolved minerals, salts, inorganic and organic elements. TDS is measured in parts per million (ppm). If we dissolved 1 gram of table salt in 1 million grams of water, this will give us a table salt solution of 1 parts per million (1ppm). 

Pure distilled water has 0ppm of dissolved solids. Very low TDS values are associated with purer water that has few dissolved minerals while high TDS values are associated with hard water. However, what makes up the TDS value is vastly more important than the value itself. Water that is low TDS could still be polluted with small amounts of a high impact element, such as copper. High TDS can be largely benign if it is mostly made up of dissolved calcium. 

In nature, the common reason why some water sources have high TDS is due to natural limestone, which is made up of soluble calcium carbonate. Limestone raises the GH, KH and TDS of water that flows through it. Low TDS water is typical for areas with no limestone, and for areas where there is plentiful rainfall (such as tropical jungles) or snowmelt (mountain side streams).

What is a normal value for TDS?

Natural water sources see a close correlation between hardness and TDS values as limestone is the main contributing factor for high mineral content water. High TDS water in nature tend to have higher GH/KH values. However, there can be outliers where water that measure high TDS actually measure low GH/KH values, so the three values can be fully separate.  

Whether or not you can grow a particular plant or keep a particular fish depends on the GH/KH readings. TDS reading alone does not provide a definitive answer.

What TDS is suitable for growing soft water plants?

Soft water plants require low KH (2dKH and below is ideal). This needs to be measured independently and cannot be extrapolated through a TDS reading. Tap water can measure above 200ppm TDS, but can still be suitable for growing soft water plants if the KH is low.

Fertilisers also increase the TDS in planted aquariums. In our own tanks, we increase the GH to 5 dGH using APT Sky. The TDS of our tanks are in the 200-230ppm range, with a KH of around 0.5 dKH. This still allows us to grow well plants that require soft water. Ultimately, its the low KH that matters where growing soft water plants are concerned, not the TDS value.

Links for further reading:

Click here to read up further details on hard vs soft water for planted tanks.

Click here to read up on how to read local water reports.

Click here to read up on temperature for planted aquariums.