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Water chemistry in freshwater fishkeeping generally refers to two difference concepts: pH and hardness. While pH is easy to test and simple to understand, hardness is quite a complex concept because it means different things in different contexts.
This is the measurement of acidity or alkalinity. The scale runs from 0 to 14, with 0 being the most acidic and 14 the most alkaline. Pure water has a pH of 7, and is consequently said to be neutral.
Most of the fishes traded in the hobby come from habitats where the pH lies somewhere between pH 6 and pH 8. All fish have a preferred pH range, and one of the factors the aquarist must consider is matching the conditions in the aquarium to those of the fishes being kept. Some examples are below:
* Angelfish, pH 6.0-7.0
* Barbs, pH 6.0-7.5
* Central American cichlids, pH 7.0-7.5
* Corydoras, pH 6.5-7.5
* Discus, pH 6.0-6.5
* Goldfish, pH 7.2-7.5
* Gouramis, pH 6.0-7.5
* Livebearers, pH 7.5-8.0
* Kribensis, pH 6.5-7.5
* Rasboras, pH 5.5-6.0
* Malawi cichlids, pH 8.0-8.5
* Tanganyikan cichlids, pH 7.5-8.0
* Tetras, pH 6.0-7.0
The pH of an aquarium is generally easy to measure using a test kit, pH-sensitive dip-strips, or electronic pH meter.
Hardness is a measurement of the quantity of dissolved minerals it contains. Soft water contains has a low mineral content, hard water has a high mineral content.
The precise mixture of minerals in the water will vary according to the sediments through which the water has flowed. Water from a chalk aquifer, for example, will have a high concentration of calcium carbonate.
Of particular interest to aquarists are Lakes Malawi and Tanganyika. While both contain hard water, the chemical compositions of the two lakes are very different. The water in Lake Malawi is richest in calcium carbonate, with smaller amounts of other salts. Lake Tanganyika, by contrast, contains hardly any calcium salts, and instead contains various magnesium, sodium and potassium salts, including carbonates, chlorides, and sulphates.
Hardness measurements: dH and KH
Two different scales are commonly used to describe water hardness, and each is measured using a particular test kit. Other systems, such as the Clarke and GH scales, may be encountered in older books but they are of no particular importance in modern fishkeeping.
°dH measures a specific mineral, calcium oxide present in the sample (though this is often correlated with a scale based on calcium carbonate concentration, particularly by British and American aquarists).
°KH measures the quantity of carbonate and bicarbonate salts present in the sample.
Most books describing basic aquarium maintenance will describe the requirements of different species of fish using the dH scale, while more advanced books, such as those focusing on aquarium plants or Rift Valley cichlids, will also use the KH scale.
General hardness: the dH scale
Each degree on the German Hardness (°dH) scale corresponds to precisely to 10 mg of calcium oxide per litre. This is used as a measure of general hardness and is a useful first approximation of the composition of the water and a key consideration when describing the requirements of a particular fish species. The following chart illustrates the connection between general hardness and water conditions (modified after Loiselle, 1988).
General hardness appears to be the critical factor for a variety of biological processes, most notably reproduction. The eggs of many egg-laying fishes only seem to develop properly within a certain general hardness range. Because it is so important, this is the essential hardness test kit for most aquarists. At the very least, you should be aware of the general hardness of your local water supply, and choose your fishes accordingly.
Carbonate hardness: the KH scale
The carbonate hardness scale is based on the concentration of carbonate and bicarbonate and is a reflection of the buffering capacity of the water. Water with a high buffering capacity resists changes in pH either up or down, resulting in very stable water conditions. This is obviously a good thing, and is essential for the maintenance of many species of fish that cannot tolerate even small pH changes, such as Tanganyikan cichlids. Excessively low carbonate hardness levels are not appreciated by aquarium plants, either, especially not those species that extract some of the carbon used for photosynthesis from the carbonate and bicarbonate salts in the water.
On the other hand, many plants as well as some fish will not tolerate very high levels of carbonate hardness either. Carbonate hardness is, along with pH, one of the two factors required for determining the amount of carbon dioxide required to fertilise a well-planted aquarium. Carbonate hardness test kits are consequently most important to advanced aquarists keeping particular kinds of fishes or trying to optimise plant growth.
The tendency of most aquaria is to become increasingly acidic over time as things in the aquarium decay. In a tank with a high carbonate hardness, this pH change is inhibited by the buffering capacity of the water. Provided you keep doing substantial and regular water changes, the pH in a tank with a carbonate hardness of 10°KH or more is unlikely to vary very much over time.
It is really only in tanks with low levels of carbonate hardness that "pH crashes" generally occur. For this reason, soft water aquaria, which usually have an acidic pH anyway, need to be especially carefully monitored. Commercially prepared pH buffering solutions that "fix" the pH at 6.0 or 6.5 are widely traded and very effective when used correctly. Typically, these contain a weakly acidic inorganic buffers that counter any slight changes in pH up or down. Even so, regular water changes are important, both to dilute the "bad" acids produced by the aquarium, and to top-up the "good" acids that provide buffering capacity.
Creating hard and alkaline conditions
Increasing the hardness and raising the pH is not difficult. The traditional approach is to use an undergravel filter in the aquarium, but using calcareous substrates instead of gravel. As the water passes through the substrate, it dissolves some of the minerals. The slow dissolution of minerals from the substrate into the water raises the hardness, particularly the carbonate hardness. The result is a system that counters the gradual acidification in the tank and maintains the hard, alkaline water that fish like Rift Valley cichlids enjoy.
A typical system would lay a bed of coral rubble or oyster shell onto an undergravel filter plate. A gravel tidy "net" would be placed on top, and then a later of coral sand would be placed on top of that. The coral sand provides a mechanical filter to stop large particles of waste from sinking into the undergravel filter. The combination of coral sand and a gravel tidy also allows the fish to root about if they want without upsetting the main body of the filter.
These systems are perhaps a bit old-school but properly set-up and maintained they work very well. As with any undergravel filter, cleaning is essential to ensure a steady flow of water, so at least once a month the coral sand at the top should be sifted and any detritus siphoned away. Once every year or two it is a good idea to dismantle the system and give it a good clean, especially if large and messy fishes are being kept. Inevitably though, bacteria and algae will sufficiently coat the grains of sand and coral rubble such that the dissolution of mineral salts into the water slows down significantly. The first sign of this is a gradual drop in pH as the buffering capacity of the aquarium declines. When this happens, you can either deep clean the substrate using hot water (or more simply) replace the old sand and crushed coral with new material.
An alternative method is to add synthetic Malawi or
Tanganyika salts with each water changes. The protocol is similar to
making up synthetic seawater: add the salts to the buckets of
dechlorinated water, stir well, and when the salts have fully
dissolved, add the water to the tank. Packages of synthetic Malawi and
Tanganyika salts can be purchased from large aquarium retailers but
some aquarists make their own salts using various "recipes"
available in African cichlid books. This DIY approach is probably not a
good idea for inexperienced aquarists, who would be better off relying
on store-bought Malawi and Tanganyika salts to start with, but
fundamentally there's no reason why a homebrew salt mix
shouldn't work very well and it will save you money in the long
A Simple But Effective Rift
Valley Cichlid Salt Mix (but it's good for Central American
cichlids, livebearers, goldfish and community fish too!)
Although plain aquarium/tonic salt (sodium chloride) is sometimes suggested as a good way to increase hardness and improve buffering, it in fact provides very little of either. Marine salt mix, on the other hand, will raise the pH and carbonate hardness quite significantly. But it also raises the salinity, something most freshwater fish do not appreciate. If you live in a soft water area and want to keep hard water fish, using marine salt mix is not really a viable option unless you are keeping salt-tolerant species, such as mollies and guppies. Rift Valley cichlids, in particular, seem to be peculiarly sensitive to salt, and elevated salinity levels have been identified as one factor responsible for the dropsy-like disease known as Malawi Bloat (Andrews, et al. 1988).
Domestic water softeners
Domestic water softeners do not produce soft water in the sense that aquarists mean. What domestic water softeners do is remove the temporary hardness (such as carbonates) that potentially furs up pipes and heaters by replacing it with permanent hardness (such as chlorides) that does not. While you can pass this softened water through a reverse-osmosis filter to remove the permanent hardness as well, until you have done so, you shouldn't consider the softened water as being suitable for soft water fish.
In fact, aquarists are divided on whether the resulting softened water is safe for keeping fish at all. The odd balance of minerals in softened water is not typical of any of the environments from which tropical fish are collected. While the chloride levels are much higher than those soft water fish are adapted to, the levels of carbonate hardness are too low for the health of hard water fishes like Rift Valley cichlids, goldfish, and livebearers. So the safe approach is not to use it in any aquarium, and instead draw water from the unsoftened drinking water source in the kitchen.
Making hard water soft is more difficult and more expensive than making soft water hard. The standard approach is to mix a small quantity of hard water with a larger quantity of pure water. This dilutes the hardness, making the water soft. The two most popular sources of pure water are reverse-osmosis (RO) filters and rainwater butts. Each has its pluses and minuses.
On the plus side, RO filters are very effective and the water they produce is not only free of minerals but also other contaminants such as nitrate. When appropriate amounts of hardening salts are added to the RO water, the result is extremely clean, safe water ideal for use in aquaria. The shortcomings of RO filters are that they are costly to set-up and maintain, slow, and wasteful.
Collecting rainwater is cheap and environmentally very sound. Unless you live in a major city or by a factory, rainwater will pick up little air pollution was it falls, so while not spotlessly clean, it is basically serviceable and safe to use. The main problems with rainwater are its unreliability since you're totally dependent on the weather and its impracticality if you don't live somewhere you can easily set up one or more water butts under the gutters. Rainwater also collects detritus as it flows through the gutters on your house, and the bottom of rainwater butts tend to accumulate rather nasty messes of decaying vegetation. This stuff isn't necessarily toxic to your fish, but it is unsightly and it will turn the water yellow. Filtering through carbon should remove the colour, and a mechanical strainer, such as a net, will remove larger particles of detritus.
Calculating the ratio of hard water to pure water required is most easily done using a mathematic trick called a Pearson Square. The picture here shows how these work (modified after Andrews, 1986). You put the hardness of the tap water in the top left corner (in the diagram here, Box A) and the hardness of the RO or rainwater into the bottom left corner (Box B). The desired hardness goes into the middle of the Pearson Square (Box C). To work out the ratio of tap water to RO/rainwater, you perform two simple calculations. The proportion of tap water is A minus C, and the proportion of RO/rainwater is B plus C. In the example shown in the diagram, you get a ratio of 5 parts tap water to 15 parts RO/rainwater, which can of course be simplified down to a ratio of 1 to 3 by dividing both sides by 5. The Pearson Square is a wonderful thing because it saves you having to mix the different waters by trial and error. By calculating the ratio up front, you can simply mix up the water on demand without any fuss or bother. It is, of course, a good idea to perform a hardness test on the result just to make sure.
Many softwater environments are also acidic, and it is normal for aquarists to create aquaria that reflect this, typically aiming for around pH 6.
The usual method is to filter the water through peat, though it is more common nowadays to use peat granulate instead. Peat granulate is a concentrated form of peat that takes up less space in the filter, leaving room for other media. Either way, what the peat does is release organic acids, in particular tannic acids and humic acids. As well as acidifying the water, these compounds tint the water to the colour of tea, producing something similar to blackwater conditions. This dark water colour dramatically enhances the colouration and visual impact of many fishes, notably things like harlequin rasboras, glowlight tetras, neons, and cardinals. These fish can often seem rather washed out in brightly illuminated tanks, but in blackwater conditions, their colours become truly luminous.
The downside of course is that darker water cuts out light penetration, so growing plants in blackwater conditions becomes more difficult. For this reason, it is often best either to skip plants altogether (often absent from real blackwater environments anyway) or else choose shade-tolerant species such as Cryptocoryne spp. and Java ferns.
A more serious problem with peat filtration is that it is very difficult to predict precisely how much a given quantity of peat will acidify the aquarium. The lack of carbonate hardness in soft water also means that the water doesn't have much buffering capacity, so sudden changes in pH are possible. Adding a pH buffer to the water and performing regular water changes is very important, especially in tanks where the carbonate hardness is below 5°KH.
You can of course use a pH buffer on its own to create acidic conditions in a soft water aquarium and skip the peat filtration entirely.
Acidity, bacteria, and biological filtration
One little-recognised effect of pH is on bacteria. For the most part, bacteria prefer alkaline pH levels. Very acidic water (where the pH is 5.5 or less) is essentially bacteria-free, and the fishes that live in such conditions are adapted to environments where the water is consequently extremely clean. When moved into less acidic environments where bacteria are more vigourous, the immune systems of such fishes often become overwhelmed and they eventually die from some opportunistic infection. Examples of fishes that demand very acidic waters for this reason can be found among the Apistogramma dwarf cichlids, Sphaerichthys chocolate gouramis, and Hemirhamphodon halfbeaks.
The problem with maintaining an aquarium at or below pH 5.0 is that biological filtration essentially stops at that point. The bacteria simply won't metabolise the ammonia and nitrite in the water quickly enough to do anything useful, so you need to rely on chemical filtration instead. Zeolite in the medium of choice under such situations.
Close: should you bother with pH and hardness?
Categorically the answer to this question is simple: fishkeeping is a lot easier if you keep fish suited to your local water conditions. Nothing trumps the ability to perform large-scale water changes on a regular basis, and if all that entails is swapping out old water for dechlorinated water taken from the mains supply, so much the better. If you live in a hard water area, then choose hard water fishes: livebearers, Central American cichlids, and Rift Valley cichlids are all ideal fish for such conditions and offer the aquarist lots of opportunities in terms of size, colour, behaviour, and breeding.
In a soft water area South American cichlids and tetras, Asian barbs and rasboras, and gouramis all make good choices. While soft and acidic water aquaria are not without their problems, these can be easily side-stepped if you simply perform large and regular water changes to pre-empt any potentially dangerous pH changes.
If you're lucky enough to have water that's only moderately hard and around neutral in pH, then consider yourself lucky: you have the most versatile sort of water and will be able to keep and enjoy a very wide variety of species.
Andrews, C. (1986): A fishkeepers guide to fish breeding. Salamander Books.
Andrews, C., Carrington, N., & Excell, A. (1988): The Interpet manual of fish health. Salamander Books.
Loiselle, P. V. (1988): A fishkeepers guide to African cichlids. Salamander Books.