Water Quality Management for Fish Farms

Ammonia / Ammonium

Ammonia is the main waste product profuced by fish and is regarded as the second most important water quality parameter after PH and dissolved oxygen in intensive fish farming. Even in relatively small concentrations, ammonia can cause significant stress and damage to fish especially bacteriological infections. In higher concentratons where the nitrogen cycle is not in balance ammonia can wipe out whole fish stocks.

Ammonia occurs in two forms: (1) Total Ammoniacal Nitrgoen (TAN) which is the toxic un-ionised ammonia (NH3) and (2) non-toxic ionised ammonia (NH4).  The pH and the temperature of the water will dictate which form ammonia is dominant at the time. When ammonia is expelled it is converted into toxic nitrite (NO2-) by Nitrosomonas bacteria. Nictrobacter on the other hand convert nitrite to non-toxic nitrate (NO3+) which in turn is taken up by plants and algae as a food source. It is therefore important to ensure that nitrite  levels are very low if not zero. If NH3 levels exceed 2ppm, fish can die.

RS Hydro provide a range of instruments to measure NH3 including handheld water quality meters with Ion Selective Electrodes (ISEs), UV-based photometers and fluorometers.

Dissolved Oxygen (DO)

Dissolved Oxygen (DO) is considered by most to be the most important water quality parameter with pH in aquaculture. Depending on the size and type of fish farm, DO levels can change very quickly indeed and can therefore endanger the lives of the fish in a matter of minutes or hours. The main problems with maintaining a suitable level of dissolved oxygen in fish farm applications is that oxygen is not very soluble in water and takes a lot of oxygenating to keep these levels; oxygen naturally diffuses to the atmosphere and the demand for oxygen is un-naturally higher in fish farm application than in the natural environment purely due to the stocking density of the fish farm.

Although fish can tolerate a drop in dissolved oxygen below 5-6 mg/L for short periods of time, it will lead to significant stress or indeed death if not increased to healthy levels. The use of handheld dissolved oxygen meters or continuous online dissolved oxygen sensors can help montior and control these situations.

pH

The recommended pH range for fish and vertebrates is between 6.5 and 7.5 as the average blood pH is around 7.4. With water passing through the fish gills and skin they are susceptible to changes in pH. Fish therefore become stressed if pH falls outside the pH6.5 - 7.5 range. pH does vary throughout the day and depends on the oxygen demand in the water and photosynthesis. CO2 is also released overnight hence lowering pH by morning but rises to its peak in the afternoon when CO2 consumption by algae (photosynthesis) is at is peak.

The pH of the water aquatic organisms live in affects the concentrations of dissolved substances, the physiological functioning of the animals, and the other plants and animals in the environment that provide food and oxygen to the ecosystem. Fish have a reduced capacity to carry oxygen if pH levels fall to critical levels but if the pH goes critically high, the portion of ammonia that is toxic also increases.

Temperature

Temperature as a water quality parameters is more important than you think as it directly affects the amount of oxygen that can be dissolved in water but also the growth of organisms and bacteria that can cause disease. Temperature also controls the rate at which food is transformed into energy which then in turn affects breathing, food intake, bodily growth and physiological behaviour. An increase in temperature will decrease the dissolved oxygen content in water which will in turn increase the fish metabolic rate and its demand for oxygen. Any organisms or bacteria also in the water will then be competing for the limited supply of dissolved oxygen hence causing stress, limited growth, stress and an increase in carbon dioxide.