Dissolved Oxygen

Dissolved oxygen (DO) is one of the most important parameters, and is simply oxygen that is dissolved in water. It gets there by diffusion from the surrounding air; aeration of water can be accelerated by tumbling over falls and rapids. It is also a by product of photosynthesis in aquatic plants.

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.

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

Although dissolved oxygen is considered one of the most important water quality parameters, levels fluctuate greatly and different species require different levels. This makes it hard to accurately assess dissolved oxygen needs but it is considered levels should stay above 5mg/l in a healthy fisheries.


Fish excrete ammonia and lesser amounts of urea into the water as wastes. Two forms of ammonia occur in aquatic systems, ionised and un-ionised. The un-ionised 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-ionised ammonia levels rise as temperature and pH increase. Toxicity levels for un-ionised ammonia depend on the individual species; however, levels below 0.06 ppm are considered safe.

The higher the pH and the warmer the temperature, the more toxic the ammonia content can be. 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.

pH (potential of hydrogen)

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 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.