Important factors that cause fish disease ( Natural Factors)

Temperature

• When water temperatures change rapidly and are too hot, fish can get disease. 
• For fry in the hatchery, the temperature variation should not exceed 2 degrees Celsius.
• When transferring fry from one pond to another, the water temperature should not differ by more than 4 degrees Celsius.
• Rapidly changes in water temperature can cause the death of entire schools of fish.

pH

• The ideal pH for fish in ponds is 7-8.5 and when the pH is less than 5 or greater than 9.5, it will cause fish death.

Dissolve oxygen 

• The appropriate amount for fish is 3-5 mg/liter of water, and when the dissolved oxygen in the water is 100 mg/liter of water, the fish will be gulping to absorb oxygen from the atmosphere. In a short time, if the dissolved oxygen level in the water does not increase, the fish will suffocate and die.

Chemical and toxic composition

• The chemical composition and toxic substances in the water in some fish ponds include substances such as lead, zinc, barium, and iron, which are found in large quantities in the soil and can also cause fish disease.
• The growth of industry, which is affected by the discharge of factory waste, has also polluted the water, causing fish disease.