Effects of Water Hardness and Dissolved Oxygen on the Environment in Aquaculture

Characteristics of Low-Hardness Water
۰Poor growth
       ۰Poor skeletal development
۰Low survival rate after molting (shedding)

Tolerance of Farmed Animals to Low Hardness
        Hardness is the sum of all divalent cations in water, mainly calcium and magnesium ions. Problems are generally more likely to occur in freshwater aquaculture than in seawater.
Hardness (mg/L CaCO3)                                        Effect

       ≤50                                                      Low pond productivity
50-100                                               High pond productivity
100-250                                             Productivity usually highest
Managing Hardness
Overview        Since the hardness of the vast majority of seawater exceeds 6000 mg/L CaCO3, there is generally no need to worry about water hardness in seawater environments.
Limestone  Applying finely ground calcium-containing limestone-type materials is usually the standard method for improving water hardness in freshwater systems.
Gypsum        Gypsum (calcium sulfate) is another commonly used material for improving water hardness. Gypsum varies in purity, but applying 1.72 mg/L of pure gypsum can raise total hardness by 1.0 mg/L. Such materials should be applied slowly to the water until the water reaches the appropriate hardness.
Mineral Feed     When mineral content in the water is low, feeding feed with a higher mineral content can also help maintain the health of farmed animals.

Characteristics of Inappropriate Dissolved Oxygen Levels in Water

Dissolved oxygen too low
۰Poor feeding by animals
       ۰Poor growth
۰Fish swim up to the surface
       ۰Not very active
۰More susceptible to disease
       ۰Shrimp molt less frequently
۰Fish gather near airstones
      ۰Shrimp congregate in shallow water areas
۰Increased opercular movement
       ۰Death
۰Drastic changes in water color

Dissolved oxygen too high
(Gas bubble disease and its symptoms can also be caused by excessively high nitrogen concentrations)
۰Bubbles under the skin
       ۰Death
۰Exophthalmos (bulging eyes)
Shrimp: rapid frantic swimming; floating at the water surface; bubbles on gills and epidermis
Tolerance of Farmed Animals to Inappropriate Dissolved Oxygen
        Tolerance levels are usually recommended values and are greatly influenced by several interacting factors. Recent feeding, high activity levels, parasites on the gills, and environmental stress caused by poor water quality can all trigger dissolved oxygen problems, even when dissolved oxygen appears to be within the suitable range. The solubility of oxygen (the amount of oxygen dissolved in water) decreases as temperature, salinity and altitude increase.
                               Effects of Dissolved Oxygen Levels on Warmwater Fish
Dissolved oxygen (mg/L)                               Effect
<0.5                          小鱼可以短时间内存活，较大的鱼死亡
<1.0                         时间延长时鱼类死亡
1.0-5.0                  The vast majority of fish can survive, but may grow slowly
>5.0                        Suitable dissolved oxygen range for aquaculture
                          
                                  Dissolved oxygen levels at which problems occur
Dissolved oxygen level (mg/L)                      Effect
0.3-1.0                           The vast majority of shrimp begin to die
1.0-2.0                           The vast majority of warmwater fish begin to die
2.5-3.5                           The vast majority of coldwater fish begin to die
3.0-4.0                           Growth of freshwater shrimp slows down
>20.0-30.0                    Gas bubble disease is likely to occur
Managing Dissolved Oxygen
Overview
Water exchange      Exchanging water with oxygen-rich water is sometimes an option, but it only applies to small water bodies and where oxygen-rich water for exchange is readily available.
Mechanical aeration     Mechanical aeration is the most commonly used method of oxygenation in aquaculture. In high-density farming systems, mechanical aeration must be used continuously, and it can also serve as an emergency measure in other situations.
Feeding      Reduce the feeding rate, because feeding increases dissolved oxygen demand.
Activity      The activity level of the animals should be reduced and environmental stress on the animals minimized. If possible, operations such as handling and capture should be avoided at this time.
Stocking density      Reducing stocking density is also a solution when dissolved oxygen problems occur frequently.
Recirculating tank systems
        Pure oxygen (liquid or gas) can be added to small water bodies, but care must be taken to avoid supersaturation of dissolved oxygen and possible gas bubble disease.
Raceway systems
Water inflow        If the water source is located higher than the pond water level, increasing inflow can raise dissolved oxygen. If the incoming water has low dissolved oxygen, reducing inflow can lower the dissolved oxygen demand associated with fish activity.
Splashing water      Before the incoming water enters the raceway, passing the water over rocks, screens or other objects can increase dissolved oxygen in the water.
Pond systems
        A certain biomass of algae and plants should be maintained in the water to prevent dangerous drastic fluctuations in dissolved oxygen, which are usually most severe at night.