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In praise of hard water

 How hard, alkaline water can be a blessing in disguise

Nematobrycon palmeri Eigenmann 1911, the Black Emperor Tetra. South America; the Rio Atrato Basin. To 4.2 cm standard length. pH 5-8, dH 5-19, temp. 23-27 C. Feeds on worms and crustaceans in the wild. Aquarium photo.

By Neale Monks

An example of a soft water organism

For many fishkeepers, the hard, alkaline water supplied to most homes in Southern England and the Midlands can seem like a curse. The water supply to homes in London, for example, can over 200 mg of calcium carbonate per litre, a hardness level comparable to that of Lake Malawi! By contrast, the majority of the tropical fish imported from South America and Asia naturally inhabit very soft water. These fish prefer water with a calcium carbonate concentration of less that 50 mg/litre. Hardness also affects pH. The hard water supplied to aquarists in London has a pH of well over 7.5, but aquarium fish such as angels, Rasboras, Corydoras, and Neons do much better if kept in tanks were the pH was below 6.5. 

It is no surprise that many aquarists use reverse osmosis filters and other devices for softening water. While undoubtedly effective, reverse osmosis are very wasteful; they typically use five to ten litres of mains water for every one litre of soft water produced for the aquarium. Wasting water on this scale is obviously bad for the environment, but couple that to the rising cost of water and the widespread use of water meters, and running a reverse osmosis filter becomes an expensive proposition. 

Collecting rainwater is a popular alterative that is ecologically sound as well as inexpensive to set up. Most garden centres sell inexpensive plastic butts for building such a system, and attaching it to the gutters on your house is a quick and usually painless process. If nothing else, the UK gets a lot of rain, so keeping a supply of stored rainwater for a small or medium sized fish tank is rarely difficult, except perhaps for a few weeks in summer. Nonetheless, using rainwater is not without problems. For one thing, any dirt or leaves in the gutter tend to wind up in the water butt, so filtering the rainwater before use is recommended. There's also a small chance that any ambient pollution in the air (such as from a nearby factory) will be dissolved in the rain as it falls. If you're concerned about this, then filtering the water through activated carbon should fix this problem. However, the biggest issue with collecting rainwater is that it is only practical if you have access to the gutters and a place to keep the water butt. This is fine if you have a house with a garden, but for anyone renting a home or living in an apartment block, collecting rainwater will most likely be impossible. 

So, is there any hope for aquarists stuck with rock-hard, highly alkaline water? Yes! While some fish and plants don't like this type of water, many do, and hard water has some unique advantages that can make the life of the aquarist much easier. To begin with, let's look at these characteristics of hard water and how they affect the lives of the fishes and plants in the aquarium. 

What makes hard water? 

Hard water is water that contains relatively high concentrations of mineral salts, in particular carbonate salts such as calcium carbonate. When water passes over or through calcareous rocks such as limestone or chalk, it dissolves some of the minerals in those rocks. In doing so, it becomes increasingly hard. Waters flowing over or through insoluble rocks, such as granite, don't pick up any hardness at all, and the resulting waters are very soft. Much of the north of Scotland gets its water from rivers of this type, and the resulting soft water is one of the factors that gives Scotch whisky its distinctive taste. 

Most of England, on the other hand gets its water supply either from rivers flowing across calcareous rocks or from underground aquifers. Aquifers can be thought of as subterranean 'sponges', porous rocks that store vast amounts of water. Aquifers sometimes come close enough to the surface for some of the water to leak out -- these are freshwater springs. Water suppliers can also drill into these aquifers and release this trapped water. About a third of all the water used in England and Wales comes from aquifers, and since this water has been sitting in calcareous rocks of hundreds, even thousands of years, it is no surprise that this water can be very hard and alkaline. 

How does hard water affect the aquarium? 

So, that's why water is hard; what does that mean for the aquarium? Besides raising the pH, hardness salts affect the animals and plants in the aquarium in a number of other ways, too. For some small fish adapted to soft water conditions, the minerals in hard water are thought to cause blockages in some of the organs. Dissection of neon and cardinal tetras has revealed damaged kidneys in specimens kept in hard water aquaria. That said, the majority of soft water fish generally do tolerably well in hard water aquaria. The problem doesn't tend to be that the fish die prematurely, though some do, but rather that it becomes impossible to get the fish to spawn or to raise the fry. Some species simply won't breed at all in hard water, while others, like Kribensis, will spawn, but the resulting fry invariably show a preponderance of a single sex within the brood. 

Another problem is that carbonate salts quickly latch onto any dissolve carbon dioxide in an aquarium, making it unavailable to any plants in the tank. In soft water aquaria, the small amount of carbon dioxide released by the fish is often enough to keep the aquarium plants happy, but in a hard water aquarium, this carbon dioxide is quickly neutralised by the carbonate salts. Carbon dioxide fertilisation therefore becomes much more important to make good the losses, and aquarists that don't fertilise this way often find that their plants only grow indifferently, if at all. 

This all sounds very negative, so what are the good things about hard water? Perhaps the biggest advantage to hard water is that it makes for a very stable aquatic environment. Over time, the tendency in most aquaria is for the pH to drop. Soft water lacks buffering salts, so this pH decline is often very rapid. Many fish that will tolerate a wide range of pH values if acclimated to them gradually can be killed by even quite small pH changes if they happen too quickly. Tetras, discus, halfbeaks, and Rift Valley cichlids are among the fishes known to respond very poorly to rapid changes in pH. Because hard water contains plenty of calcium carbonate and other hardness salts, these mop-up the acidic chemicals and slow down the pH change. In fact, the average fishkeeper with a hard water aquarium will probably never even notice a change in pH because the water itself will buffer any potential changes so effectively. 

Another big plus to hard water is that some aquarium plants can use the carbonate salts as a fertiliser. Plants adapted to soft, acidic waters like many Cryptocorynes and some Echinodorus rely on dissolved carbon dioxide as carbon source for photosynthesis. They effectively take the carbon dioxide and turn it into sugars and all the other organic chemicals they need for energy and growth. This is essentially the same thing as land plants do when they absorb carbon dioxide from the atmosphere. However, some aquatic plants can absorb the carbonate salts and strip away the carbon from them, and use that as their carbon supply. 

The list of plants capable of doing this includes many that do very well in aquaria, including Ceratophyllum demersum, Cryptocoryne becketti, Echinodorus bleheri, Egeria densa, Elodea canadensis, and Vallisneria spp -- all popular and easy to obtain species. If you have hard water and don't want to be bogged down with carbon dioxide fertilisation, then these are definitely the plants for you! Admittedly, some of these plants are fussy in other ways. Echinodorus bleheri, for example, needs a rich substrate and good, strong lighting, but Ceratophyllum demersum and most of the Vallisneria are adaptable and easy to keep. If you want to keep livebearers, then Ceratophyllum demersum is difficult to beat as a floating plant that provides a refuge for newly born fry. Egeria densa, on the other hand, is a sturdy, fast-growing species ideally suited to subtropical tanks. Vallisneria spp. are perhaps the most versatile aquarium plants, and few aquarists haven't grown these plants at some point. 

Fish for the hard water aquarium 

Perhaps the biggest myth about hard water is that most tropical fish don't like it. Nothing could be further from the truth: while it is true that many kinds are adapted to soft, acidic water conditions, there are still plenty of colourful and entertaining species that positively thrive in hard, alkaline water. Perhaps the group of fish that best exemplifies this are Central American livebearers, in particular the families Poeciliidae and Goodeidae. Other examples of fish that do well in hard water include many kinds of cichlid, most of the freshwater gobies and sleepers, some of the rainbowfish and killifish, and a very large proportion of the oddballs -- things like freshwater soles, halfbeaks, glassfish, and puffers. What's missing from the list are things like tetras, barbs, and catfish, and for the most part the popular species of these do indeed prefer soft, acidic water. Nonetheless, even from these groups there are definitely species that prosper in hard water, and of these, more will be said later on. 


Among the livebearers, the Poeciliidae are pre-eminent, at least as far as popularity goes. These include the guppies, platies, swordtails, and mollies, all of which make excellent choices for the hard water aquarium. In recent years, the Endler's guppy has become especially popular, and while aquarists debate whether the commercial stock is truly 100% Endler's guppy or some mix of Endler's and regular guppy, there's no question these brightly coloured, active little make an excellent contribution to the community aquarium. Whichever guppy you choose, bear in mind that they are small and easily bullied. They shouldn't be kept with more aggressive species or substantially larger ones that might view them as food. Pufferfish in particular seem to have a taste for the long fins of male guppies. 

Platies are a particular favourite of mine; they're peaceful, brilliantly coloured, and generally easy to keep. Platies are available in almost every colour imaginable, including shades or red and blue that wouldn't look out of place in a reef tank! Swordtails are only a little less varied, but tend to be much more outgoing and fun to watch. Male swordtails can be a bit aggressive though, and it's definitely a good idea to keep twice as many females as males. Mollies round of the 'big four' livebearers, and at lengths of up to 15 cm, adult female mollies are easily the largest livebearers many aquarists ever encounter. 

Livebearers that are more unusual are the Goodeids. Whereas poeciliids are ovoviviparous, Goodeids are truly viviparous. What's the difference? Ovoviviparous fish merely retain their eggs inside the body, whereas viviparous fish actually supply the eggs with food via an umbilical cord. Though certainly not as frequently seen as guppies or mollies, at least two species, Ameca splendens and Xenotoca eiseni, can be obtained easily enough from the larger aquarium stores. Ameca splendens, the butterfly Goodeid, is a peaceful and hardy fish that is often said to be the ideal Goodeid for the beginner because it slots into a community tank very well. As its common name suggests this is a pretty fish, with a speckled, silvery body and, on the male, a black tail edged with yellow. Xenotoca eiseni is also a pretty fish and one that does well in hard water, but wild specimens in particular can be persistent fin-nippers. Tankmates should be chosen with care, and be sure and choose species that do not have long, trailing fins. 

Another oddball livebearer worth considering is the wrestling halfbeak, Dermogenys pusilla. These smaller relatives of the popular Celebes halfbeak thrive in hard water and make wonderful surface dwellers for any community of small fishes. Like Goodeids, these are viviparous fish, and breeding generally presents few problems. 


The cichlids have to be close to the top of any list of hard water fish. While many are adapted to soft, acidic water conditions (not least of all the ever-popular angels, rams, and discus), by far the majority prefer hard, alkaline water. All the cichlids from the Rift Valley lakes -- Lakes Malawi, Tanganyika, and Victoria -- fall into this category, as do many of the species from Central America and from Asia. Now, having said this, not all of these cichlids make good community fish. So while keeping a Rift Valley cichlid aquarium is certainly one option for the aquarist in a hard water area, the focus of this article is on those species that will do well in a community setting. On the other hand, a tank set up specifically for these fish can be extremely rewarding. These fish range from tiny, shell-dwelling forms rather like gobies through to giant predators and schooling plankton-eaters. Any aquarist living in a hard water area should certainly consider keeping these 'freshwater coral reef fish' -- in terms of colour, activity, and variety, Rift Valley cichlids are hard to beat. 

One example of a Rift Valley cichlid that can work well in the community tank is Neolamprologus brichardi. Being relatively large and distinctly predatory, it shouldn't be kept with very small tankmates such as guppies, but larger fish, like Sailfin mollies and rainbowfish, will work well. Because they form large, active schools when kept in big tanks, Neolamprologus brichardi are often seen in public aquaria. Virtually all the fish offered for sale are commercially bred and consequently inexpensive, but there is one issue to bear in mind before putting a few into your tank. Should these fish decide to breed they can be disruptive. Like all cichlids, they extend considerable care to their eggs and fry, and in doing so may terrorise any tankmates not smart enough, or fast enough, to keep out of the way. 

African riverine cichlids are usually much easier to keep in community tanks. One popular choice is the Kribensis, Pelvivachromis pulcher. Kribs are very attractive fish, the best examples sporting a mix of cream, purple, and brown. While they are just diligent as about broodcare as other cichlids, they tend to hold quite small territories and often the only sign that the fish have spawned at all is when the aquarist finds the parents leading their offspring out on a hunt for food. Note, however, that hard water isn't ideal for breeding Kribs because a pH above neutral tends to result in a preponderance of male fry. 

One Asian cichlid that will do well in hard, alkaline water is the orange Chromide, Etroplus maculatus. They are actually brackish water fish, but the amount of salt they need is so low that they can easily be mixed with many species of fish as well as hard water tolerant aquarium plants. A specific gravity as low as 1.002 is all that is required, equivalent to a concentration of about four grammes of salt per litre. Guppies, mollies, halfbeaks, bumblebee gobies, and glassfish will all thrive in such conditions. Orange Chromides are available in two forms, the greenish wild type, and a bright orange artificial form. The 'orange' orange Chromide is very popular and easy to obtain, and it adds a flash of brilliant colour to any community tank. Like all cichlids, Chromides can be territorial and a bit disruptive if they choose to breed, but otherwise this species is as tolerant and peaceful as you could hope for. 

Central American cichlids tend to be a little on the aggressive side to make good community fish, though many aquarists have kept convicts, Archocentrus nigrofasciatus, and firemouths, Thorichthys meeki, in mixed species tanks. They are relatively large fish, the convict being about 10 cm long when mature and the Firemouth over 15 cm, so any decision to keep these fish in a community tank will depend on the aquarium being large enough (at least 120 cm long). Firemouth cichlids in particular generally get along well with fast-moving, surface dwelling species such as Sailfin mollies and rainbowfish, though they will eat small fish like guppies. Even so, it's a bit of a gamble, and more than a few aquarists have seen a peaceful firemouths turn into psychopathic thug seemingly overnight. 

Having mentioned rainbowfish a few times, it's worth saying a little about them generally. In some ways, they're the hard water equivalent of the tetras and barbs; though many species normally inhabit neutral, even soft, water in the wild, they do uncommonly well in hard, alkaline water. Several species are simply stunning fish; Melanotaenia boesemanni and Glossolepis incisus immediately spring to mind as some of the loveliest freshwater fish available to aquarists. They are also rather long-lived and hardy, and once settled in and feeding, rainbowfish tend to be completely problem-free. About the only downside to rainbows is that they tend to be a bit more expensive than the average barb or tetra. Juvenile Melanotaenia boesemanni, for example, seem to go for around £3 to £5 a throw, and you really want to keep at least half a dozen if you want these fish to look their best. A bit less expensive are fish like Melanotaenia mccullochi and Melanotaenia fluvialitis, not to mention the lovely 'dwarf' rainbowfish Melanotaenia praecox

Closely related to the rainbows are the atherinids like the Celebes rainbow, Telmatherina ladigesi, and the Madagascar rainbow, Bedotia geayi. The former is a silvery fish with yellow fins and a neon tetra-like blue stripe along its flanks; a school if these makes a stunning addition to any hard water community tank. The Madagascar rainbow is perhaps a little less showy but still a lovely fish, with a blue or purple band along the body and bright yellow fins. Both of these fish are hardy, inexpensive, and easy to keep. 

Glassfish, Parambassis ranga, also make good schooling fish for hard water aquaria. These fish naturally inhabit a range of waters from soft and acidic through to slightly brackish, and adapt well to most water conditions. They are hardy, easy to keep fish except for one thing; they tend to be very fussy about food. Live or frozen foods are readily accepted, in particularly frozen lobster eggs and bloodworms. On the other hand, glassfish are lively and entertaining, and when settled in have an almost cichlid-like personality that encompasses periods of playfulness, chasing, and begging for food. 

A significant number of killifish inhabit hard water, but only a few of them routinely are traded as aquarium fish. Perhaps the best known of these is the Florida flag fish, Jordanella floridae. The common name of these fish, incidentally, refers to the fact they look like the American flag, while hailing from the state of Florida, and not that they look like the Florida state flag. These killifish are easy to look after, though males can be aggressive towards others of their kind. Treat them like dwarf cichlids, and don't overcrowd them. Another hard water killifish is the lamp-eye, Aplocheilichthys macrophthalmus. This African fish is very small and cannot be kept with anything much bigger than a platy, and usually does well only when kept with other, similarly sized species. Endler's guppies, for example, would make good tankmates, as would bumblebee gobies and glassfish. Lamp-eyes are not the easiest killifish to keep and breed, but they are very attractive, and make an excellent choice for the aquarist with space only for a small aquarium. 

Rounding off the hard water egg-layers are the gobies and sleepers, in my opinion terribly underrated aquarium fish. Gobies and sleepers are small, full of personality, and often brightly coloured as well. Perhaps the only reason they don't sell well is the belief that they are difficult to keep. Consider the bumblebee goby -- a little jewel that wouldn't look out of place in a reef tank, but inexpensive, hardy, and long-lived (when kept well). Bumblebees are of course fussy about their food, and will not accept flake, but frozen foods such as bloodworms and lobster eggs, as well as live Daphnia, are greedily accepted. Another wonderful goby is the candy stripe goby, Awaous flavus. This is a big goby, up to 10 cm long, and so needs a bit more space than the average goby, but it is definitely worth it. Besides being a very pretty fish, it has bags of personality and quickly becomes tame enough to feed by hand. New kinds of goby and sleeper turn up every year, so definitely keep your eyes open for these, since most will do very well in hard water aquaria.

Hardwater-tolerant tetras 

As a rule, the popular South American tetras tend to tolerate rather than thrive in hard water. Some, like Neons, cardinals, and Glowlights, suffer somewhat, and their mortality in hard, alkaline water can be very high. Nonetheless, a few tetras do inhabit hard water streams and rivers, and these make excellent choices for the aquarist with a hard water aquarium. One of the best is the x-ray tetra, Pristella maxillaris, a pretty, peaceful tetra that adds colour and movement to any community of small fishes. It isn't a fin-nipper, and so can be trusted with things like guppies, and is big enough that it isn't at risk of being eaten by things like halfbeaks or dwarf cichlids. Another fine choice for the community tank is the blind cave tetra, Astyanax mexicanus. Because this fish inhabits streams in limestone caves, it is perfectly adapted to hard, alkaline water. It is, of course, a wonderful oddball fish, and despite having no eyes it has an uncanny way of navigating and finding food very effectively; a splendid fish for the aquarist after something different. 

Brackish water fish 

To finish with, mention has to be made of the brackish water fishes. These fish thrive in hard water, and in fact for most of them, the harder, the better. Their exact needs in other regards vary considerably and fall outside the scope of this article, but for the experienced aquarist, puffers, moray eels, soles, and pipefish can all make excellent subjects for a single-species tank. Scats, monos, and archerfish are more amenable to community life, but being rather large, you will need a large aquarium to keep even these fish on their own, let alone with a variety of tankmates. 

I hope this article has made it clear that just because you don't have soft water in your aquarium doesn't mean you can't keep many different kinds of interesting and colourful fish. New types of cichlid, goby, and livebearer are continually appearing on the market, and many of these make excellent subjects for a hardwater community tank. If your prepared to do some research and specialise, Rift Valley cichlids and brackish water fish both offer challenges and rewards all their own. In short, don't see hard, alkaline water as an obstacle, but as an invitation to keep some of the most interesting fish in the hobby!

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