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The Soft Water Aquarium: Risks and Benefits

By Neale Monks


Soft water is water that contains low levels of dissolved minerals. Aquarists consider what that has less than 5Ë° general hardness to be "soft water" and between 5-10Ë° general hardness "moderately soft water". It is also quite common for soft water to be described in terms of calcium carbonate concentration, in which case soft water has less than 50 mg/l calcium carbonate and moderately soft water between 50 and 100 mg/l calcium carbonate. 

Soft water exists wherever water is flowing across terrain that is poor in soluble minerals. The rivers of South America, Southeast Asia and West Africa are predominantly rivers of this type. Because so many of the ornamental fish sold in the trade come from these areas, many hobbyists assume that they need or prefer soft water conditions in the aquarium. Up to a point this is indeed the case, but creating and maintaining a soft water aquarium places an extra set of challenges on the aquarist. These are as follows:

1. Soft water exhibits a stronger tendency towards pH instability than hard water. All aquaria become more acidic over time, but in soft water aquaria this trend can be very rapid. Since few fish will tolerate rapid changes in pH, frequent pH tests and the use of chemical buffers are an important aspect of maintaining a soft water aquarium.

2. If you don't have soft water on tap, turning hard water into soft water is expensive. Reverse-osmosis filters provide soft water, but at a price. Collecting rainwater is a practically zero-cost alternative, but it is not without problems of its own.

3. Domestic water softeners do not produce soft water usable in an aquarium. All they do is replace limescale-forming minerals with minerals that don't form limescale. This is fine for washing machines and dishwashers, but bad for fish tanks.

4. Filter bacteria work best in hard, alkaline water conditions. In very soft and acidic water, filter bacteria may not work at all, forcing the aquarist to use less efficient methods of filtration, such as the use of zeolite.

5. Not all livestock will do well in soft water conditions. Most notably, the livebearers or Central America will not do well in soft water aquaria and will be plagued with problems such as fungal infections and finrot.  

Preamble: Why Bother?

One of the paradoxes in freshwater fishkeeping is that while most fish naturally from soft water environments will thrive in hard water aquaria, the reverse is almost universally not true. Tetras, Barbs, Gouramis, Corydoras catfish and Angelfish are all examples of originally soft water fish that are routinely and successfully kept in hard water community tanks. But Livebearers, Central American cichlids and Rift Valley cichlids almost never adapt to soft and acidic water conditions. In other words, if all you want is a mixed community tank, then hard and alkaline water will allow you to mix Platies, Neons and Corydoras without problems.

So why bother with soft water aquaria at all? Soft water becomes useful in one of two situations.

In the first place, soft water may be critical to breeding a certain fish. While tetras and South American dwarf cichlids will live quite happily in a hard water aquarium, they will not spawn. Sometimes they will deposit their eggs willingly enough, but because of the improper water chemistry, the eggs will not develop. In other instances they simply won't exhibit any breeding behaviours at all. For aquarists maintaining community tanks this isn't an issue, but for hobbyists wanting to breed these fish, then transferring them to a soft water aquarium becomes essential.

Rams are classic soft water fish, and generally don't do well in anything other than soft water aquaria

The second situation where a soft water aquarium becomes important is when certain delicate fish are being kept that cannot adapt to hard water. These tend to be species kept by advanced hobbyists, including things like wild-caught Discus and Apistogramma, certain Rasboras, and some of the more demanding Gouramis such as Chocolate Gouramis.  

Precaution: Acidification

Before looking at how to set up a soft water aquarium, it is essential to first explain the problem of acidification. All aquaria have a tendency to become more acidic over time. Acidification is caused by four main things:

1. Nitrification. Filter bacteria convert ammonia excreted by fish and from decaying organic matter into nitrite first and then nitrate. Nitrate forms nitric acid, and this in turn lowers the pH. In most freshwater aquaria, there is not active removal of nitrate (or nitric acid) through denitrification, because the anaerobic conditions required tend not to be favoured by freshwater aquarists. Photosynthesising plants will of course remove some nitrate, which they use as a nitrogen source for protein synthesis, but except in heavily planted aquaria this effect is usually trivial.

2. Respiration. As organisms respire they produce carbon dioxide, including the fish, plants and filter bacteria. The carbon dioxide dissolves in the water to form carbonic acid, the same stuff that makes soda pop acidic. To some degree vigourous aeration will release the carbon dioxide into the atmosphere, but a heavily stocked tank will still experience more rapid acidification than a lightly stocked one.

3. Photosynthesis. The effect of plants on pH is complex because aquatic plants use two different sources of carbon, dissolved carbon dioxide and bicarbonate ions. Dissolved carbon dioxide forms carbonic acid, which lowers pH. As plants photosynthesise, they remove this carbonic acid, allowing pH to rise. However, in most aquaria the actual concentration of carbon dioxide in the water is very low, which is why aquarists concerned with plant growth need to fertilise their tanks with addition carbon dioxide. If there is insufficient carbon dioxide for their needs, plants will switch to using up bicarbonate ions. Bicarbonate ions are a major part of the alkalinity reserve in the water, that is, the ability of the water to resist changes in pH. With the bicarbonate ions taken away, the water loses some of its buffering capacity, and the pH drops. The overall result is that rapid plant growth coupled with insufficient carbon dioxide and/or bicarbonate concentration can lead to dangerously fast acidification.

4. Bogwood and peat. Though often overlooked, bogwood can be a significant source of acidity. This is especially true where the bogwood has not been fully cured. The acids produced by bogwood are predominantly tannins. Peat also effects water in a similar way. If peat is used as a substrate (as is sometimes done with killifish) the water will become more acidic.

While all aquaria will be subject to some or all of these factors, not all aquaria react in the same way. A critical difference between those tanks that become acidic rapidly and those that do not is the alkalinity reserve. Hard water (whether fresh, brackish or marine) contains an abundance of mineral ions, in particular carbonate and bicarbonate ions. These form the alkalinity reserve. Any acids produced within the aquarium will be neutralised by the carbonate and bicarbonate ions.

In hard water aquaria, the alkalinity reserve outweighs potential sources of acidity

Soft water aquaria are different. By definition, soft water contains few dissolved minerals, and as a consequence the alkalinity reserve is very low. There's nothing much in soft water to mop up acidic chemicals, so these will accumulate and lower the pH between water changes. In other words, soft water aquaria are subject to faster and more severe drops in pH than hard water aquaria.

In soft water aquaria, insufficient alkalinity allows rapid acidification

The problem is that fish do not like rapid changes in pH. When pH changes, fish will adjust the chemistry of their blood to prevent physiological problems. But having to constantly alter their blood chemistry is stressful. It is therefore important that as well as providing a suitable pH for the fish being kept, the aquarist keeps that pH steady. Moreover, once the pH drops below 6.0, fish not otherwise adapted to strongly acidic conditions will suffer from acidosis. This results in damage to the gills and skin, problems with respiration, and eventually death. Below pH 6.0 the bacteria in a biological filter essentially stop working, leading to potential ammonia poisoning. Even for those fish tolerant of acidic conditions, the sudden rise in ammonia can be lethal.  

Setting up a soft water aquarium

Obviously calcareous materials such as seashells and tufa rock cannot be used in a soft water aquarium. These will dissolve and raise the hardness and pH.

It is also important to avoid using anything that promotes acidification. As a general rule, the soft water aquarium needs to be as chemically inert as possible, containing nothing that will either raise or lower the pH and hardness. Bogwood, peat and coconut shells are best left out of the soft water tank because they produce tannins and remove hardness. To avoid problems with carbonic acid, understock the tank and use aeration do drive the carbon dioxide into the air. Likewise nitric acid is best managed through understocking the tank, feeding the fish only sparingly, and performing regular water changes to dilute the nitrate. While plants can be used in the soft water aquarium, it isn't a good idea to use large quantities of rapidly growing species, particularly species that extract bicarbonate from the water.

One way to avoid problems with acidification is to ensure that there is a modest alkalinity reserve in the water, and then to use substantial water changes to replenish it every week. Unless you are keeping species that need very soft water, moderately soft water conditions will suit most Tetras, Rasboras, Apistogramma and Discus. Aim for a hardness between 5-10Ë° dH and a pH around 6.5.

Cardinals do not need particularly soft water to thrive, even though in their natural habitat the water has almost no mineral content at all

Alternatively, you can use a pH buffer to stabilise water chemistry. A chemical buffer neutralises chemicals that threaten to raise or lower the pH. Used properly, they can be very effective and dramatically improve the stability of the aquarium. The commercial chemical buffers offered to aquarists typically contain phosphoric acid, and stabilise the pH around 6.5.

The value of pH buffering products is often misunderstood. Adding a buffering product that slightly acidifies the water doesn't turn hard water into soft water. Neither will fish that need soft water be satisfied with hard water that has been acidified using a chemical buffer. The value of a buffer is in making soft water stable; nothing more, nothing less.

Filtration can be a problem. Filter bacteria work best at around pH 7.0, and as the pH drops, their performance sharply decreases. Below pH 6.0, filter bacteria are hardly working at all. Chemical filter media that remove ammonia directly (such as zeolite) are the only option in such aquaria. Because chemical filter media need to be replaced or recharged, regular water testing and routine filter maintenance are essential. The idea is to change the filter media before ammonia is detectable -- not afterwards!  

Making soft water

If you live in a soft water area you may have water of appropriate pH and hardness for a soft water aquarium. Maintenance of the aquarium will be relatively straightforward because you can perform large and regular water changes, and thereby minimise any problems with water quality of acidification.

For aquarists in hard water areas things are more complicated. Soft water needs to be made up by mixing a small proportion of hard water with a larger proportion of pure water. This will dilute the hardness in the tap water resulting in something with a pH and hardness level useful for soft water fishkeeping. There are two main sources of pure water: reverse-osmosis (or RO) filters, and rainwater. Each source has its pros and cons.

RO water is convenient because you can produce pure water as required. RO filters come in different sizes scaled to the demands of different fishkeepers. However, RO filters are expensive to buy and certain components may need replacing at intervals. RO filters are also rather wasteful, with about 10 litres of tap water being discharged for every 1 litre of pure water produced.

Rainwater can be collected and stored for use in aquaria, but it is important to clean the water before use

Setting up a rainwater butt to collect water from the gutters on your home is easy to do and the equipment very inexpensive. Obviously the rain itself costs nothing. The downside to rainwater use is that it depends on regular rainfall, something not all aquarists can rely upon. It is also important to store a certain amount of water to allow for water changes during dry spells. The risk of air pollution contaminating rainwater is pretty trivial, and simple filtration through carbon will remove most potential pollutants anyway. Detritus on the roof and in the gutters is more of a problem. While decaying leaves and drowned insects don't really harm the fish directly, they can contribute to acidification, and they are certainly unsightly. Regular cleaning of the entire system is therefore an essential and tedious chore, and a net should be used to sweep up any detritus in the rainwater before use.  

Pearson's Square

To calculate the ratio of pure water to hard water required to make soft water of a given hardness you need to use a Pearson's Square. The diagram below shows how this works. In this example shown, the ratio of tap water to pure water is 5 parts to 15 parts, i.e., one part tap water to every three parts pure water. Alternatively, you can use tools like my freebie program Soft Water Ware to do these calculations for you.

Pearson's Square is quick way to calculate the proportion of hard water and pure water to create soft water of required hardness


Because soft water is poor in minerals, various trace elements supplements are produced for aquarists keeping soft water aquaria. These supposedly help the health of the fish, and are simply added to each new batch of water. Whether or not they are useful will depend on how soft the water is that you are using. If you are mixing pure water with hard water to create something around the 5-10Ë° dH mark suggested, there is unlikely to be any shortfall in the amount of these trace elements available. But if you are keeping your fish in water that is softer than this, then adding mineral supplements may make sense.

Once the pure water and hard water are mixed, additives such as blackwater extract can be used to provide trace elements the fish require to do well

Some blackwater extracts also include trace element supplements and are intended to be used in the same way. The value of blackwater extract is often misunderstood by inexperienced aquarists. Adding blackwater extract to a hard water aquarium doesn't turn it into a soft water aquarium. Blackwater extract by itself merely changes the colour of the water. It's a cosmetic effect, and while safe to use and visually attractive, shouldn't be mistaken as a tool for keeping fish that need a genuinely soft water aquarium.  

Peat filtration

Peat used to be a very popular way to soften and acidify water for use in aquaria. It has largely fallen out of favour, particular since the advent of RO filters. Peat acts as an ion exchange resin, remove minerals from the water and replacing them with organic acids known as humic acids. Besides softening the water, peat turns the water dark brown in much the same way as blackwater extract.

The main problem with peat is that it isn't completely predictable. Adding peat directly to the aquarium (for example inside a canister filter) will soften and acidify water over time, but the rate at which it operates will depend on the original hardness of the water. Water that is very hard may not change very much at all, while water with relatively low hardness can become acidic so rapidly the fish are stressed.

Peat is best used to soften and acidify water prior to use in the aquarium

The safest and simplest approach is therefore to use peat to treat water before adding it to the aquarium. Peat is placed inside a box or canister filter, and that filter placed in a container of water. As the water is passed through the filter, the peat will reduce the hardness and lower the pH. Once the water has reached its desired hardness and acidity, it can be used in your soft water aquarium. This may take many days though, even weeks, depending on how hard the water was to begin with and how much peat you are using. So you will need to experiment a bit to get a system that produces enough water to meet your demands.  

Not an option: Domestic water softeners

Contrary to their name, domestic water softeners do not make soft water usable in aquaria. What they do is produce water that doesn't contain the temporary hardness that furs up pipes. They do this by replacing the temporary hardness minerals with other types of minerals, typically sodium salts. Water from a domestic water softener is not really suitable for use in any aquarium, let alone a soft water aquarium.  

Closing remarks

The soft water aquarium presents both opportunities and hazards to the fishkeeper. On the plus side, there's no doubt that some of the real jewels of the freshwater side of the hobby -- including Apistogramma, Harlequin Rasboras, Cardinal Tetras and African Killifish -- never look better than when kept in soft, acidic water stained the colour of tea. Under such conditions these fish truly rival mbuna and coral reef fish in their beauty, whereas when kept in hard water their colours are often lacklustre and muted.

But the soft water aquarium is categorically not easy to set up or maintain. Besides the problem of making soft water if you don't have it on tap, there are serious issues with acidification and management of the nitrogen cycle. Neglecting these issues will quickly lead to disaster.  

Further reading

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