Question

Why CaO is added in water body before pisciculture?

Answer 1

It neutralizes the water by increasing the pH of water(normally acidic)

Explanation:

{I suppose the topic should be biology.}
Liming helps prevent wide swings in pH. So, Liming may enhance the effect of fertilization. Liming also adds calcium and magnesium, which are important in animal physiology.

Fluctuations in pH are the result of the interplay of photosynthesis and respiration. Nighttime respiration increases `CO_2` concentrations, creating carbonic acid and causing pH to fall. During the day phytoplankton absorb `CO_2` for photosynthesis, causing pH to rise.
Large, daily changes in pH can stress aquatic animals . The desired alkalinity for many animals is 50 mg/L or higher. Liming to raise total alkalinity to the required or preferred ranges buffers the water and reduces swings in pH.