Water chemistry is undoubtedly one of the most confusing aspects of the hobby, something which has been compounded by inconsistency, generalisation, over-complication and over-simplification in much of the literature available to the aquarist. As a result, this topic is a regular discussion point in the forum. The main focus of this article will be on water hardness and pH. I will aim to give the beginner an introduction to these two areas, and also to give the more experienced aquarist a greater understanding of their water and how to successfully change it, if desired.
Hardness in its simplest form is the amount of dissolved (or suspended) minerals in the water. As rainwater passes through rocks and soil, it picks up these minerals. The quantity and type of these minerals depends of the type of the rocks the water comes into contact with. For example, water in a limestone catchment will be rich in minerals, while in a granite area it will be very poor in minerals. As a general rule, sedimentary rocks contain more chemicals that affect water chemistry than igneous and metamorphic rocks, which tend to be more inert. You will generally know whether you live in hard water area by the amount of limescale on your kitchen and bathroom appliances. Soft water is water which is low in mineral content.
As most aquarists know, pH is a measurement of how acidic or alkaline (basic) the water is. Be careful not to confuse this meaning of basic with the other meaning of basic, as in simple. pH is a measure of the concentration of hydrogen ions (H+) in the water. An ion is a chemical particle with an electric charge. A positive charge is shown as +, while a negative charge is represented with -. pH is measured from 0 to 14, with 0 being most acidic, and 14 being the most basic. The scale is logarithmic, meaning that a decrease of one unit means a ten-fold increase in acidity, and a decrease of two units means a one hundred-fold increase in acidity. A pH of 7.0 is neither acidic nor basic, and is known as "neutral". The best way to measure pH in the aquarium is with a good quality narrow range test kit, or with a frequently calibrated electronic meter.
The majority of catfish are found in lotic (river) habitats. The three main types of tropical river water (blackwater, whitewater & clearwater) have been discussed in this previous Shane’s World article. Rivers are often dynamic, rapidly changing habitats, and can change enormously over the course of a year. During the tropical dry season, water levels can be very low, temperatures very high, and the concentration of minerals in the water also high. After a period of heavy rain, these conditions can change within hours as the river floods. The water may become much cooler and softer very quickly (this is often when a lot of species begin to breed). As a result, most catfish are highly adaptable to changing water conditions.
Given this fact, why do most people worry so much about providing exact water conditions for their catfish? The truth is, in most cases they don’t need to. The majority of fishkeepers have very hard alkaline tapwater, akin to that found in lake Tanganyika (600 µS/cm & pH >8). However, such is their adaptability, most catfish can thrive in these conditions, even if their natural home is generally soft and acidic. It is a general consensus among aquarists that pH and hardness are very much of secondary importance in keeping your catfish healthy. In the limited size of the aquarium, keeping a stable pH and hardness is preferable to fluctuations. Rather than experimenting with acids etc, it is much better to allow the catfish to slowly adapt to your water. Instead, concentrate your time on making sure the water is low in nitrates, of the correct temperature, and the catfish have an appropriate diet. However, there are always exceptions, and this is where the aquarist must do their homework. Some blackwater species are not as adaptable as their white and clearwater cousins. An example of this is Ancistrus dolichopterus, a sought after loricariid from the Rio Negro. These fish will not fare as well as other Ancistrus spp. in hard tapwater. While most catfish can be maintained in hard tapwater, breeding them will often require much softer, more acidic water. This is where we must start to alter our water, but first it is important to know how we measure hardness in the aquarium. The measurement of hardness is extremely confusing; with probably as many different measurements as there are L numbers. I will try to ignore as many of these as possible, and concentrate upon the most common and useful measurements.
The concentration of minerals in water has a direct influence on the amount of electricity the water can conduct. Conductivity is measured as microsiemens per centimetre (µS/cm). Electronic conductivity meters are available to the aquarist, and are well worth the money. These meters can also express conductivity as "total dissolved solids" (TDS) in ppm (parts per million), which is the same as mg/l. TDS is an expression of conductivity as a concentration. Divide conductivity (µS/cm) by two to get TDS. Conductivity/TDS gives the best measurement of hardness, as it includes all types of minerals, not just the main common ones.
Minerals & Ions
There are hundreds of different minerals in most aquarist’s tapwater, mostly in trace amounts, but the most common water hardening mineral is calcium carbonate (CaCO3). This compound comes from limestone rich rocks and soils. When dissolved in water, the compound dissociates into two parts: the calcium (Ca2+), and the carbonate (CO32-). Together with magnesium (Mg2+), the calcium part provides "permanent" hardness to water. It is called permanent hardness, as it cannot be removed from water by boiling/adding acids, unlike the carbonate, or "temporary" hardness. Other common metal ions such as sodium (Na+) and potassium (K+) do not contribute to towards permanent hardness, as they only have a single positive charge. They do however contribute towards conductivity. Most GH test kits test for the concentration of calcium and magnesium ions only (permanent hardness).
Most people are familiar with the term GH (general hardness), but depending on who one reads, it can mean a variety of things. Some authors refer to GH as the sum of permanent and temporary hardness (i.e. Ca2+, Mg2+ and carbonates), while others call GH the total concentration of all minerals in the water (i.e. the same as conductivity or TDS). Most people and test kits however, refer to GH as just the expression of permanent hardness (Ca2+and Mg2+). GH can be expressed as mg/l (milligrams per litre), or as degrees (shown as either dGH, °GH or dH°). To get from mg/l to degrees, multiply by 0.056.
Returning to the example of calcium carbonate. As already discussed, the Ca2+ part provides permanent hardness, and the CO32- part is the carbonate. Carbonates provide temporary hardness or KH (from the German; Karbonahärte). This is also known as alkalinity or buffering capacity, but carbonates are only really part of alkalinity. Alkalinity is the concentration of basic chemicals in the water. Bear in mind that even at an acidic pH, there will still be a quantity of these basic chemicals in the water, it is just much less than at a higher pH. Carbonates (CO32-) are negatively charged ions, and are able to accept protons from positively charged acid (H+) ions. This is how they "neutralise" an acid. Water with a high alkalinity has a very stable, but high pH (like Lake Tanganyika). As the alkalinity drops, the pH will become lower, but less stable, and more liable to be altered by acids produced in the aquarium (tannic acids from bogwood for example). Carbonates have different forms, depending on the pH of the water. At pH values above around 10.5, carbonates form the majority (>50%) of the buffering capacity. They are strongly basic, as each CO32- is able to accept two H+ ions. Each reaction forms a bicarbonate ion (HCO3-). As pH gets lower, the carbonates are converted gradually to more and more bicarbonates. These bicarbonates form the greatest part of the buffering system in the aquarium. Bicarbonates become exhausted at around pH 5.0, as they are converted to carbonic acid (H2CO3). This is the point where the water can lose all its capacity to neutralise acids, and a dangerous pH crash can occur, often killing fish. However, carbonates are not the only substances able to provide alkalinity. Phosphates (as H2PO4-) for example can be part of the buffering system at low pH values. Many pH buffers on the market contain phosphates. These are often best avoided though, as high levels of phosphates can cause algal blooms, as it is also a powerful fertiliser. KH test kits work by adding acid to a water sample, and changing colour once set pH is reached. As a result, they will show total alkalinity (from phosphates and borates etc), not just from carbonates, so the title "KH test kit" is a little misleading. This is why it is much better to refer to the results from these kits as alkalinity, rather than KH. Like GH, carbonate hardness can be expressed as mg/l or as dKH. The correct way to express it (as alkalinity), is as milliequivalents per litre (mEq/l), but this is unlikely to catch on among aquarists.
With this mind boggling array of measurements for hardness, it can be hard to decide what the best method to use is. For the experienced aquarist, a conductivity meter is the most effective way to accurately measure mineral content. However, when maintaining a soft water tank with a low pH, it is essential to also make sure the water has sufficient buffering capacity. A conductivity meter cannot measure this accurately, so a KH test kit is also a must. If you only have chemical GH and KH test kits, the results from each can be added together to give a rough approximation of TDS. A conductivity test on some typically hard UK tapwater gave a TDS of 315mg/l. The combined results of the GH and KH test kits gave a reading of 330mg/l. In this case it is quite accurate, but it must be pointed out that elevated levels of sodium or phosphate for example, will affect the accuracy of this method.
How To Change Hardness
If you do decide to change your water’s mineral content, what are the best methods to achieve success? For the aquarist that has poorly buffered water, or wishes to keep species that like harder water such as the rift lake Synodontis spp., raising the mineral content is very easy. Simply add a mineral rich substance such as aragonite, tufa rock or coral chips to the aquarium. If these are not appropriate in the tank, bags of the material of choice can be included inside the filter. The rocks will slowly dissolve in the tank, keeping the water buffered. The quantity needed will vary with the waters’ initial softness and acidity.
The aquarist that wishes to provide soft water has a much more difficult time. Methods such as adding peat, oak leaves or bogwood to the tank, are often recommended. Unfortunately, these methods are usually limited in their success. The high levels of alkalinity found in most tapwater can easily buffer the tannic acids released from these materials. Peat will slightly reduce the alkalinity, and the pH may fall slightly, but this often requires amounts so large that the water becomes heavily stained, and the fish no longer visible. Peat will also not reduce the conductivity by any significant amount, so the water is not truly soft. However, once water has been demineralised by other methods, peat filtering can be used to naturally reduce the pH, and provide the characteristic blackwater stain.
Fish that require soft water, or need a soft water trigger to begin breeding, best respond to the change in total mineral concentration. This is why domestic water "softeners" (that replace Ca2+and Mg2+ for Na+ and K+) may prevent your washing machine from furring up, but do not provide suitable soft water for fish, as the water still has a significant mineral concentration. The only reliable methods of getting soft water are via distillation, reverse osmosis, deionisation and rainwater (which is naturally soft and acidic). Distillation is not an option to the home aquarist, as there is no suitable equipment available, and energy costs would be prohibitively high. The most popular method of acquiring soft water is with a reverse osmosis unit. Reverse osmosis (RO) removes around 95% of the minerals and chemicals in the water. These units work by forcing water through minute pores in a membrane. It acts, in effect like a microscopic sieve. The "pure" water is collected in a container and waste water is flushed away. Unfortunately, these units waste a huge amount of water for only a small amount of "pure" water produced. The ratio will vary depending on the TDS of the tapwater, with very hard water producing vast amounts. Obviously these units are not suitable for people in areas of the world where water is in shortage. The alternative to RO is deionisation (DI). DI units use resins to remove all ions from the water, producing purer water than with RO. There is also no wastage with DI. However, most hobbyist units run with single use resins. These are expensive, and rapidly exhausted in hard water. If the aquarist wants soft water for a small breeding project, they are usually able to buy RO or DI water from their LFS. Another popular method of acquiring soft water is by using rainwater. Many people are put off using rainwater by reports of air pollution. However, when compared to the substances found in tapwater, rainwater is definitely the lesser of the two evils. Make sure the water butt or container is very clean, as there may be a high bacterial count. Filtering the rainwater through activated carbon may also be beneficial.
Demineralised water, due to its lack of alkalinity, is not suitable for direct use in the aquarium. It is true that natural blackwater is softer than even RO water, but in nature, the huge volumes of water mean that any seasonal changes are gradual enough for fish to adapt. Therefore, a small amount of minerals will need to be returned to the demineralised water to make it suitable for use in the home aquarium. Many aquarists just add tapwater back into the demineralised water to reach the desired parameters. To achieve a pH between 6.0 and 7.0, an alkalinity of between 1dKH and 3dKH is usually needed. If the pH needs to be lowered, reduce the proportion of tapwater, or add a small amount of peat. The only problem with the tapwater method is if the aquarist’s tapwater is very high in nitrates, then the reconstituted water may also have more nitrates than desired. The alternative is to use a commercial mix of salts and buffers to achieve the same results. It is important to know what is in these products, and to add them to your water change water, and not direct to the tank. As mentioned above, it is best to avoid phosphate buffers, unless you are confident with their use. Carbonate based buffers are the best choice. To achieve consistent results, it is best to warm (to desired temperature) and aerate the water for twenty-four hours (to let the pH stabilise) before adding to the tank. When maintaining a soft water tank, it is essential to test pH and alkalinity weekly to be confident that the water is appropriately buffered. It is also essential, when moving fish, to make sure you know the pH and hardness of both types of water. It is best to let fish acclimatise very slowly to new water. The greater the change, the longer they should acclimatise. Slowly drip new water into the bag with an airline. Several hours is not too long.
With this information, I hope the users of this forum can enjoy more success with keeping and breeding their catfish.
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