WATER CHEMISTRY DEFINITIONS

WATER CHEMISTRY DEFINITIONS FOR USE IN AQUACULTURE

Managing the chemistry and quality of water used for aquaculture can have an enormous effect on the well-being of fish, and therefore the productivity of ponds.

The following definitions and explanations are supplied to help understand more fully, some of the terminology used in aquaculture.

Total Hardness / General Hardness (GH)

Short Definition:

Dissolved calcium and magnesium.

Explanation

Total Hardness is a measurement of both permanent & temporary hardness; i.e. compounds of calcium and magnesium, including bicarbonates and carbonates, expressed as calcium carbonate in mg/L. A few other ions add to water hardness, but they are usually not present in significant quantities.

Hard water can be recognised by the difficulty of getting soap to lather.

In aquaculture, hardness levels can have a significant effect on hatch rates, growth, and overall well-being of fishes.

Fish can absorb calcium, which is essential for growth, directly from the water.

Total hardness is frequently referred to as General Hardness (GH) in aquarium literature, and on test kit labels.

Total hardness test kits usually measure hardness in parts per million (ppm), which for our purpose, is essentially equal to milligrams per litre (mg/L).

European test kits generally measure hardness in German degrees of hardness (dH).

Each German degree is equivalent to 17.9ppm

Total hardness levels of between 20-300ppm are considered acceptable for pond fish culture, but many species can tolerate levels well in excess of this.

Temporary Hardness / Carbonate Hardness (KH)

Short Definition:

Carbonates and bicarbonates of calcium and magnesium.

Explanation

Temporary Hardness is the same as Carbonate Hardness (KH).

It is that part of total hardness that is caused by calcium and magnesium being dissolved by carbonic acid (carbon dioxide in water); in other words carbonates and bicarbonates of calcium and magnesium.

These compounds are the most common cause of alkalinity in natural waters.

Water with a high KH is considered strongly buffered and resists becoming acid.

A good example of temporary hardness is seen in the common problem of “scale” in water pipes, hot water services, and kettles, in hard water areas.

This “scale” is a direct result of calcium and magnesium salts precipitating out of solution as carbon dioxide levels drop, usually as a result of water being heated.

Aquatic plants utilize dissolved carbon dioxide as their primary source of carbon during photosynthesis.

Once the dissolved carbon dioxide has been used up, carbonates and bicarbonates become the source of carbon.

At night and on cloudy days the process is reversed as plants produce carbon dioxide and the carbonate and bicarbonate balance is restored.

This process of using and generating carbon dioxide causes the pH of water to fluctuate, however under most circumstances the presence of temporary hardness will moderate the severity of the pH swings. (See “use of lime” below)

KH or alkalinity test kits usually give a good indication of carbonate hardness levels (see below for details)

Temporary hardness / carbonate hardness levels of between 20-200ppm (even up to 300ppm) are considered acceptable for pond fish culture.

Potential problem with KH test kits.

Most KH test kits actually measure total alkalinity, of which KH is only a part.

These kits use an acid to titrate alkalinity in water samples; this measures the waters capacity to neutralize acid.

Alkalinity is usually similar to KH because the majority of alkalinity in natural waters is caused by carbonate hardness.

But it is possible to have water with a high alkalinity that contains little or no carbonate hardness.

For example, if one adds an alkaline phosphate to water as a buffer, the alkalinity increases and the KH test kit will give you a higher reading, but the true KH (temporary or carbonate hardness) will not have increased.

KH test kits are still a very valuable tool, but it’s important to remember they measure total alkalinity, and really should be labelled this way.

Many people rely on establishing the carbon dioxide level of water by using the commonly available table showing the relationship between pH, KH, and carbon dioxide. As explained previously, if “basic salts” (hydroxides), other than carbonates and bicarbonates of calcium and magnesium are present, the true KH can’t be established using a normal (KH) alkalinity test, and therefore the carbon dioxide values, as shown on the chart, become inaccurate.

Temporary Hardness (true KH) can be accurately measured by the following method.

1/ Measure Total Hardness (GH)

2/ Then boil the original sample for 5 to 10 minutes (boil sufficient water so a large proportion does not evaporate). Cover tightly so as to exclude carbon dioxide from the air, and allow sample to cool.

3/ Allow any precipitation to settle (or filter it out).

4/ Measure GH again. (This second measurement is the Permanent Hardness)

5/ Subtract the second reading from the first (Permanent Hardness minus Total Hardness) = Temporary Hardness (true KH)

Permanent Hardness

Short Definition:

Salts of calcium and magnesium, other than carbonates and bicarbonates.

Explanation

Permanent hardness is also referred to as non-carbonate hardness; it is the salts of calcium and magnesium that cannot be removed from a solution by boiling.

As explained previously, a few other ions add to water hardness, but are usually in insignificant quantities.

Permanent hardness of water is ascertained by boiling the sample as described in steps 2, 3, & 4 above. It is the Hardness or GH reading, taken after boiling.

Measuring permanent hardness of aquaculture water is normally un-necessary, but is mentioned here for completeness.

Alkalinity / Buffering Capacity

Short Definition:

The capacity of water to neutralize acid.

Explanation

Water with a high alkalinity is likely to contain ions of bicarbonates, carbonates and hydroxides.

It has a strong buffering capacity, and is much less likely to undergo sudden pH drops or violent pH fluctuations.

On the other hand, it is difficult to lower the pH of large volumes of such water if the need arises (although it can be done using strong acids).

That part of alkalinity that is made up of carbonate and bicarbonate salts of calcium and magnesium, is known as temporary hardness, or true carbonate hardness (KH), as determined by the boiled water method described above.

Dissolved carbon dioxide will cause pH of water to drop, but the alkalinity / KH will remain the same.

Alkalinity (or a KH reading) is one of the most important water quality parameters to measure as it shows the underlying alkalinity of water, excluding the effect of carbon dioxide.

In other words, pH will drop as carbon dioxide increases, but alkalinity (KH) reading remains relatively constant.

Expelling the carbon dioxide, say through aeration, will allow the pH to rise, returning to its starting point.

If substantial quantities of aquatic plants and algae are present, water with a high alkalinity but low temporary hardness, is likely to become dangerously alkaline during periods of high photosynthesis (carbon dioxide removal by algae and plants). See note on use of lime below.

Alkalinity is determined by a test involving acid titration (the method employed in most KH test kits).

The units of measurement are parts per million (ppm), or milli-equivalents per litre (meq/L). One meq/L = 50ppm. Alkalinity levels of 20-300ppm are considered acceptable for pond fish culture.

Short definition:

A measurement of how strongly acid or basic (alkaline) water is.

Explanation

pH is measured on a logarithmic scale of 0 to 14, with the centre point 7.0 considered neutral.

For every decrease of one degree, the acidity increases ten fold. E.g. a pH of 5.0 is ten times more acid than pH 6.0

Water in aquaculture systems, if not sufficiently buffered, will tend to become acid as a result of biological processes.

Water with a high alkalinity or high KH will naturally tend to be basic, and resist becoming acid.

See notes on alkalinity / KH above, and use of lime below.

The pH of ponds is usually lower in the morning, due to increased carbon dioxide levels, and higher late in the day, due to uptake of carbon dioxide by plants and algae.

pH can be tested using simple aquarium test kits, dip and read test strips, or electronic testers or meters. (Electronic meters must be calibrated regularly).

Fish kept at a pH of less than 6 and over 9, are likely to be stressed, grow more slowly, and be susceptible to disease.

What does the p and the H stand for? (From google)

pH stands for the potential of hydrogen or power of hydrogen.

H (Uppercase): Stands for the Hydrogen ion

The higher the hydrogen ion concentration, the more acidic the water is, resulting in a lower pH value.

p (Lowercase): Represents a mathematical operator denoting the negative logarithm (base 10) of the quantity.

It is believed to derive from the German word "potenz" or French "puissance," both meaning "power" or "potential," originally used to express the "hydrogen ion exponent." (Remember at the beginning of the pH explaination I said it is measured on a logarithmic scale.)

USE OF LIME (In ponds)

Agricultural lime, which is mostly calcium carbonate, is highly recommended for use in earth ponds where pH is low (below 6) and alkalinity is low, under 20 or even 40ppm.The solubility of calcium carbonate increases as the pH decreases.

Liming raises low pH levels, and minimises dangerously high pH spikes due to plant material removing carbon dioxide during photosynthesis in waters of low temporary hardness. Under the above mentioned conditions, if for example sodium bicarbonate is used as a buffer, the pH may rise to dangerous levels of 11 or more.

Generally, lime should be used for earth ponds, and shell grit or coral rubble for tanks.

The use of lime (and fertilizers) has been shown to substantially increase productivity of ponds with low alkalinity. Ammonia becomes more toxic in alkaline water, and this needs to be considered before fertilizers are added, or any changes are made to pH.

Detailed instructions on liming and fertilizing are beyond the scope of this article, and the reader is recommended to refer to appropriate literature.

Total Dissolved Solids (TDS) & Osmoregulation

Short definitions:

TDS is a measurement of all mineral substances that have dissolved in water. (Sometimes referred to as ionic strength)

Osmoregulation is the physiological process that maintains the proper balance of salts between the inside of the fish and the water it is living in.

Explanation

Most fish cannot survive for long in distilled water, and most freshwater fish cannot live in sea water.

Between these extremes we see that fresh water fish have a variable tolerance for dissolved minerals.

Therefore keeping fish in water of the appropriate TDS level for the species, will minimise effort required by the fish to osmoregulate, and therefore minimise stress.

In waters of low ionic strength, the addition of sodium chloride (common salt) has been commonly used for quickly and safely raising TDS for holding, treating, and transporting fish.

Testing TDS, (sometimes measured as conductivity) is the only practical method for quickly and efficiently getting a good indication of the level of all salts and minerals dissolved in water.

This is an extremely useful test, and electronic TDS testers or meters, are usually very reliable, economical, and easy to use.

As with all electronic testing equipment, they need to be calibrated as specified.

TDS levels ranging between 100 and 2000ppm are considered suitable for grow out, and most aquaculture applications, but many species can be cultured at levels much higher than these.

Oxygen (O2) or Dissolved Oxygen (DO)

Short definition:

Oxygen sustains all aerobic life, and its availability is a critical factor in aquaculture.

Explanation

Oxygen, which makes up 20.9% of air, diffuses slowly from the atmosphere into water.

Agitating the water and creating ripples at the surface by means of aeration or paddle wheels is a proven method of increasing dissolved oxygen and expelling carbon dioxide.

Plants and algae also contribute to oxygen levels during daylight hours due to photosynthesis.

But plants also respire, consuming oxygen and producing carbon dioxide, which can adversely affect ponds at night.

For every 1 gram of oxygen consumed by fish, 1.4 grams of carbon dioxide is produced.

To sustain fish life, sufficient oxygen must be available in the water at all times, to enter the fishes’ blood through the gills.

Fish held at low oxygen levels have been shown to be more susceptible to disease, feed less, and grow more slowly.

Low oxygen may be experienced from some underground water sources, under overcrowded conditions, in a polluted environment, after an algae crash, during hot or still and cloudy weather (particularly at night) and during transportation.

Dissolved oxygen levels can be measured on site, using chemical test kits, but using the more expensive electronic oxygen meters, is considered a better method.

Fish gasping at the surface often indicates oxygen depletion.

The solubility of oxygen reduces as temperature of water rises and salinity increases.

Seawater contains approximately 25% less oxygen than freshwater.

Oxygen levels in pure water at saturation are about 11ppm at 10 degrees C, and 8ppm at 25 degrees C.

Levels approaching 4ppm are considered stressful, and between 4 and 0ppm deadly. (Depending on the species.)

For normal health and growth, oxygen levels should be kept above 5ppm, and preferably close to saturation.

Biological Oxygen Demand (BOD)

Biological oxygen demand measurements are generally used to ascertain the degree of water pollution.

BOD is a measurement of how much oxygen is consumed in a given water sample, held at 20 degrees C over a fixed time.

This test is normally carried out by specialised water testing laboratories.

High BOD indicates high levels of waste biological products (organics) e.g. high levels of bacteria, algae, zooplankton etc. BOD measurements are not often used by fish farmers.

Knowing the oxygen level, and what percentage that is of saturation, is much more useful information.

Carbon dioxide (CO2)

Short definition:

Carbon dioxide accumulates in water as a result of respiration by aerobic life.

Explanation

When fish respire, carbon dioxide leaves the blood and enters the water through the gills.

It then accumulates, or is taken up by plants and algae (by photosynthesis during daylight hours), or expelled at the water / air, interface. Aeration, (agitating the water) helps expel dissolved carbon dioxide into the atmosphere.

Because dissolved carbon dioxide competes with oxygen for space in fishes’ blood, CO2 levels of 20-30ppm reduce the oxygen carrying capacity of fish blood by up to 50%.

Dissolved carbon dioxide forms carbonic acid and in un-buffered water can reach a pH as low as 4.5.

As previously mentioned, water with high carbonate hardness is considered well buffered, and is less likely to undergo dramatic pH swings.

High or toxic concentrations of free carbon dioxide are seldom found in alkaline surface water (above pH 7.0).

Carbon dioxide is neutralized in the presence of bicarbonates or carbonates and is then not toxic to fish.

Bicarbonates and carbonates act as a “carbon dioxide storage unit”.

In daylight, plants take up the stored carbon dioxide and at night it is replaced, due to the respiration of fish, plants, and other aquatic life.

Carbon dioxide may reach dangerous levels in underground or bore water, in heavily stocked acid water, in transport water, and at night or on cloudy days in ponds with high levels of algae or aquatic plants.

Decomposing organic matter (i.e. algae crash) is another major source of CO2 in ponds and aquaria.

Free carbon dioxide can be measured using chemical test kits, or can be ascertained from the commonly available chart showing the relationship between carbonate hardness, pH, and carbon dioxide. (If the KH is high, and the pH is low, carbon dioxide is usually high)

Carbon dioxide levels above 40-50ppm are considered dangerous, particularly if oxygen levels are low.

Lots more to learn

This is by no means an exhaustive list of terms used in aquaculture.

Fish farmers can gain enormous benefits by understanding the environment our aquatic life inhabits.

Knowing when, and how to alter just one or two factors, can swing a farm from struggling to success, in merely a season or two.