Water Chemistry

Water chemistry can seem intimidating at first, but once a few basic principles are memorised, it becomes quite easy. The parameters of prime concern are dissolved oxygen, temperature, pH, alkalinity, hardness, and carbon dioxide. Optimum water conditions must be maintained at all times to minimise stress. Crowding, dissolved oxygen depletion, rapid changes in temperature, incorrect feeding and osmotic imbalance are all well known causes of stress and must be avoided. Sub-optimum conditions, while not immediately lethal, may stress the fish, resulting in delayed mortality or failure to grow.

Everyone involved in fishkeeping should invest in water test kits. An excellent water quality management program will result in fewer fish disease problems, better growth and less use of chemical treatments. The cost of test kits will pay for themselves many times over; both in numbers of fish saved and increased enjoyment of the hobby. For aquarium keeping, the high precision of sophisticated analytical methods is not needed to make informed management decisions.

Intensive stocking in recirculating systems and aquariums requires frequent monitoring. If fish are maintained at high densities, then temperature, dissolved oxygen, ammonia, nitrite, and pH should be monitored regularly. At lower stocking densities, water quality parameters can be monitored less frequently. However, regardless of the frequency, monitoring should be conducted at a standard time of the day. The time of measurement and observed values should be recorded, as good record keeping is essential for successful fishkeeping.

Alkalinity and hardness can be measured less frequently, perhaps once a week, as they do not fluctuate as rapidly. Carbon dioxide should be monitored closely and the means to correct problems should be readily available. It is preferable to monitor dissolved oxygen early in the morning, when conditions stressful to fish are most likely to occur. Conversely, temperature and pH in ponds are best measured during the late afternoon. Chlorine or Chloramine levels in tap water should be determined so that corrective measures can be initiated.

Numerous test kits are available on the market from which you can choose, but they can vary in quality and price. You are therefore advised to select test kits carefully and not buy the first one you see on the shelf. Outdated reagents are a major source of inaccurate tests resulting from kits that have a limited shelf life. If used beyond their shelf life, then they will not give accurate, reliable results.

Some manufacturers print use-by dates on their kits but unfortunately, these manufacturers are in a minority. If you have a test kit without a use-by date, and you have had it for over six months, then you should replace the test chemicals. Test kits containing dry reagents are usually superior to kits that contain liquid reagents. Liquid reagents are generally less expensive but over time become unstable. Reagents in a dry form generally remain stable for longer periods. The most stable way of storing dry reagents is in sealed foil pouches; this protects them from oxygen, moisture, and light until they are used. Several aquarium test kit manufacturers have adopted this type of packaging.

Alkalinity
Alkalinity is not the same as hardness. Calcium and Magnesium are primarily responsible for hardness. However, in most waters, alkalinity and hardness have similar values because the carbonates and bicarbonates responsible for total alkalinity are usually in the form of Calcium carbonate or Magnesium carbonate. However, waters with high total alkalinity are not always hard, since the carbonates can be in the form of Sodium or Potassium carbonate.

A desirable range of alkalinity for rainbowfishes should be maintained higher than 50 mg/L (ppm) at all times, with levels up to 200 mg/L. Alkalinity in excess of 200 mg/L won't adversely affect rainbowfishes, but it can interfere with the action of certain commonly used aquarium chemicals. Alkalinity can remain relatively constant in ponds, but will decrease steadily in non-supplemented aquarium systems. Adding buffering compounds to ponds or aquarium systems will increase the alkalinity and stabilise the pH. The pH should always be monitored during alkalinity increases, as a high pH increases the toxicity of unionised ammonia.

The units used to measure alkalinity will depend on the test kit. Some use milliequivalents (mEq/L),
°dKh (degrees of carbonate hardness), mg/L or parts per million (meaning ppm of calcium carbonate equivalents). mEq/L stands for milliequivalents per litre.

For aquarium purposes, you can use the following conversion factors:

1 mEq/L = 2.8 °dKh = 50.04 mg/L (ppm) CaCO₃
1 °dKh = 17.9 ppm CaCO₃

Carbon Dioxide
Carbon dioxide is a by-product of decomposition of organic matter, fish, and aquatic plant respiration and can cause problems for most rainbowfishes if levels build higher than 20 mg/L. Most can tolerate concentrations of 10 mg/L provided dissolved oxygen concentrations are high. Well-maintained aquariums normally contain less than 5 mg/L of free carbon dioxide. Although, levels may fluctuate daily from 0 to 15 mg/L.

Problems with carbon dioxide are only likely to develop in the aquarium when rainbowfishes are maintained under somewhat crowded conditions or if being added as a fertiliser for aquatic plant growth. Carbon dioxide can build up to significantly high levels in systems with large numbers of fish and relatively slow water turnover. Adequate aeration or surface agitation, and buffering of the water will keep carbon dioxide at acceptable levels. Adequate buffering will initially remove free carbon dioxide and store it in reserve as bicarbonate and carbonate buffers. Small water exchanges will also reduce the levels of carbon dioxide.

Dissolved Oxygen
In general, it is recommended that the dissolved oxygen concentration be kept near to saturation. Typical values in a healthy aquarium system should be 8 mg/L or 85-95 %Saturation at 24°C. In aquariums equipped with proper filtration and aeration, insufficient dissolved oxygen is seldom a problem. It is therefore generally unnecessary to test oxygen in aquariums. However, corrective measures need to be initiated if conditions become unfavourable. The only way to know for sure if low oxygen levels have caused fish deaths in an aquarium is to measure the oxygen in the water when the deaths occurred.

There is little information available on how rainbowfishes are affected by low levels of oxygen. The minimum dissolved oxygen level that rainbowfishes can safely tolerate depends upon individual species and temperature. Different life stages (i.e., eggs, larvae, juveniles and adults) may also have different oxygen needs.

Water Hardness
The water hardness of natural freshwaters throughout Australia and New Guinea differ greatly, and the required water hardness for rainbowfishes in captivity is just simply not known. However, based on the relatively few studies that have been conducted, it would appears that levels of 60-250 mg/L as calcium carbonate, are desirable for best survival, growth, and feed conversion. Some euryhaline species (such as Pseudomugil cyanodorsalis) maintained in freshwater may have a higher requirement.

pH
In Australia and New Guinea the pH of freshwater streams naturally varies between catchments due primarily to differences in catchment geology and vegetation. However, in general, most freshwater streams have a pH range of 6.5 to 8.0. There are, of course, exceptions to this general rule. Coastal streams generally range from about 3.9 in tannin-stained streams associated with coastal 'wallum' heath, to 8.8 in streams at the headwaters of some catchments. Forested areas and coastal areas with high rainfall generally have the lowest pH. Naturally low pH also occurs due to seasonal wetting and drying of peaty soils in wetlands and waterways. In areas with highly alkaline subsoils natural pH is generally in the range of 7.0-8.5.

There is no definite pH range for maintaining rainbowfishes in captivity, but a gradual deterioration of their health is likely as the pH values are removed from their preferred range. Rainbowfishes will survive reasonably well in waters with a pH range of 6.0 to 8.5. If pH readings are outside this range, growth is reduced; their slime coat can suffer, making them susceptible to disease. At values below 5.0 or above 9.0, mortality, impaired growth and reproduction can be expected. From my own experience, most rainbowfishes in captivity do not seem to be comfortably in water below pH 6, certainly not for any extended period. There may be some isolated populations that have adapted to extreme conditions as low as pH 4.0. However, a pH range of 6.5 to 7.8 is suggested for maintaining rainbowfishes in captivity.

Temperature
Rainbowfishes can tolerate a fairly wide temperature range and have been kept for fairly long periods at 22~30°C. A widely used standard temperature for keeping them is 24~25°C. Heat-shock can occur at temperatures above 36°C. A general maintenance temperature of 22~24°C is acceptable while an increase to 28°C can be used for breeding purposes.

Rainbowfishes, unlike mammals, are unable to regulate their body temperatures and are thus subject to the temperature in their surrounding environment. Temperature regulates their metabolism and their need to take in nutrients. Therefore the temperature of the fish's environment is a major and even the deciding environmental factor in determining growth rate, metabolism, and nutritional efficiency. In fact, temperature will influence all biological and chemical processes in an aquarium.

Rainbowfishes characteristically display a considerable range of growth rates, depending on conditions such as food, space, numbers, competition and water temperature. In tropical waters, which have prevailing high temperatures, rainbowfishes generally grow faster, mature younger, and have a shorter life span than rainbowfishes in temperate waters. Rainbowfishes do, however, display a wide range of sizes, growth rates and life spans in captivity.

In their natural environment, many rainbowfishes are exposed to temperatures that vary considerably. This is dependent on the size and depth of the body of water, water flow, whether exposed to the sun or in shaded rain forest streams, the time of day and the season. A deep shaded backwater, for example, can be strikingly cooler than a shallow section that is exposed to the sun. The overall range has been reported from a low of 5° to a high of 38°C, and even higher in a shallow body of water exposed to full sun at midday. Temperatures in these water bodies commonly fluctuate by 8 to 12°C each day. However, these temperatures are conditions that wild rainbowfishes have to be able to endure to survive and are not recommended for aquarium keeping.

In their natural environment, rainbowfishes can search for more favourable conditions by moving into cooler or warmer water. However, in captivity this is not possible. Each species has a preferred or optimum temperature range where it grows best. At temperatures above or below optimum, their growth is reduced and mortalities may occur at extreme ranges.

It should be noted that the survival rates for rainbowfishes in their natural habitats declines sharply when the water temperature is high and will often die at temperatures above 36°C. Such increases in temperature are common in tropical waterbodies of Australia during the late dry season. Although, the cause of death is more likely to be lowered oxygen levels rather than the higher temperature.

© Copyright Adrian R. Tappin
Updated June, 2011.