Oxygen measurement is often presented in two forms, as a percentage saturation figure (% sat.) or as an amount of oxygen per (milligram) per unit of water (litre) this is often shown as mg/l or p.p.m.

The amount of oxygen that is dissolved in water is critical, different species of fish require a great deal more than others, salmonids such as the Atlantic salmon (Salmo salar) and the Brown trout (Salmo trutta) require much more than cyprinid fish such as the carp (Cyprinus carpio) and the tench (Tinca tinca).

The amount of oxygen that is in solution as dissolved gas can vary significantly for numerous reasons. The weather in the form of low pressure can lead to de-oxygenation especially in times of thunder storms. Oxygen can be depleted by organic pollution and by the die back of algae following a bloom. The amount of oxygen that is taken from water by organic pollution is often shown as its biochemical oxygen demand (BOD).

Plants are both a source of oxygen usage and oxygen productivity in aquatic system, they produce oxygen during the day and use it during the hours of darkness, this diel rhythm can often be responsible for low dissolved oxygen levels before dawn during the warmer summer months.

pH is the measurement of hydrogen ions (H⁺) in water, it is the concentration of these H⁺ ions that will determine if water is acidic or basic (basic is the correct term for alkaline). The scale for measuring the degree of acidity is called the pH scale, which ranges from 1 to 14. A value of pH 7 is considered neutral, neither acidic or basic; values below 7 are considered acidic; above 7, basic. The acceptable range for fish is normally between pH 6.5-9.0. The ideal range for cyprinid fish is above pH 7.0 at around pH 7.5- 8.0.

Fish excrete ammonia and lesser amounts of urea into the water as wastes. Two forms of ammonia occur in aquatic systems, ionized and un-ionized. The un-ionized form of ammonia (NH₃) is extremely toxic while the ionized form (NH⁴⁺) is not. Both forms are grouped together as "total ammonia." Through biological processes, toxic ammonia can be degraded to harmless nitrates. Un-ionized ammonia levels rise as temperature and pH increase. Toxicity levels for un-ionized ammonia depend on the individual species; however, levels below 0.06 ppm are considered safe.

However, the intermediate form of ammonia, known as nitrite has been known to occur at toxic levels (brown-blood disease) in fish ponds.

The danger ammonia poses for fish depends on the water’s temperature and pH, along with the dissolved oxygen and carbon dioxide levels.

The higher the pH and the warmer the temperature, the more toxic the ammonia. Also, ammonia is much more toxic to fish and aquatic life when water contains very little dissolved oxygen and carbon dioxide. Ammonia is toxic to fish and aquatic organisms, even in very low concentrations. When levels reach 0.06 mg/l, fish can suffer gill damage. When levels reach 0.2 mg/l, sensitive fish like trout and salmon begin to die. As levels near 2.0 mg/l, even ammonia-tolerant fish like carp begin to die. Ammonia levels greater than approximately 0.1 mg/l can indicate polluted waters.

In the presence of oxygen a naturally occurring species of bacteria Nitrosomonas spp. converts ammonia into nitrite (NO₂^-). This process is the first step in the conversion of ammonia to nitrate (NO₃^-), it is known as nitrification. Nitrite is much less toxic than ammonia. Levels above and in the range of 10 –20 mg/l are lethal to many species. Nitrite toxicity decreases as the availability of mineral salts increases.

The process of denitrification continues as Nitrobacter spp. of bacteria oxidise nitrite into the less toxic nitrate ion (NO₃^-). Nitrate is toxic to fish at levels above and in the range of 50-300 mg/l depending on the specific fish species.

Nitrate is a major constituent of farm fertilizer and is necessary for crop production. When it rains, varying nitrate amounts wash from farmland into nearby waterways. Nitrates stimulate the growth of plankton and water plants that provide food for fish. This may increase the fish population. However if too much nitrate is present the process of eutrophication will be advanced. In such circumstances algae and water plants grow wildly, choke the waterway, and use up large amounts of oxygen. Many fish and aquatic organisms may die as a secondary effect.

Nitrates also may get into waterways from lawn fertilizer run-off, leaking septic tanks and cesspools, manure from farm livestock, animals wastes (including fish and birds), and discharges from car exhausts.

The element phosphorus is necessary for plant and animal growth. Nearly all fertilizers contain phosphates (chemical compounds containing the element, phosphorous). When it rains, varying amounts of phosphates wash from farm soils into nearby waterways. Phosphates stimulate the growth of plankton and water plants that provide food for fish. This may increase the fish population and improve the waterway’s quality of life.

However if too much phosphate is present the process of eutrophication will be advanced. In such circumstances algae and water plants grow wildly, choke the waterway, and use up large amounts of oxygen. Many fish and aquatic organisms may die as a secondary effect, despite phosphates not being directly toxic to fish.

Phosphates enter waterways from human and animal wastes (the human body releases about a pound of phosphorus per year), phosphate-rich rocks, wastes from laundries, cleaning and industrial processes, and farm fertilizers. Phosphates also are used widely in power plant boilers to prevent corrosion and the formation of scale.

Phosphates exist in three forms: orthophosphate, metaphosphate (or polyphosphate) and organically bound phosphate. Each compound contains phosphorus in a different chemical formula. Ortho forms are produced by natural processes and are found in wastewater.

Poly forms are used for treating boiler waters and in detergents; they can change to the ‘ortho’ form in water. Organic phosphates are important in nature and also may result from the breakdown of organic pesticides, which contain phosphates. Some values for total phosphate-phosphorus are given

Amount of total phosphate-phosphorus – mg/l	Effects
0.01 – 0.03	Amount of phosphate-phosphorus in most uncontaminated lakes
> 0.025	Accelerates the eutrophication process in lakes
> 0.1	Recommended maximum for rivers and streams

Water hardness is similar to alkalinity but represents different measurements. Hardness is chiefly a measure of calcium and magnesium, but other ions such as aluminium, iron, manganese, strontium, zinc, and hydrogen ions are also included. When the hardness level is equal to the combined carbonate and bicarbonate alkalinity, it is referred to as carbonate hardness.

Hardness values greater than the sum of the carbonate and bicarbonate alkalinity are referred to as non-carbonated hardness. Water hardness is inextricably linked to a waters ‘buffering’ capacity.

A buffering capacity is essential in aquatic systems that are to support healthy and viable fish populations as well as many other aquatic life forms. It is essential to avoid wide swings in pH.

Without some means of storing carbon dioxide released from plant and animal respiration, pH levels may fluctuate in ponds from approximately 4-5 to over 10 during the day. Calcium carbonate can achieve this by binding with the H⁺ions. Generally speaking the greater the water hardness the more buffering capacity available in the water. Hardness values of at least 20 p.p.m. should be maintained for optimum growth of aquatic organisms.

To book your eleven parameter water quality analysis please telephone 01403 820999 for a purpose made sample bottle. For a fully inclusive price of £60.00 you will receive the analysis results with an accompanying short explanation.