**Explanation:**

**Introduction:**
Water tanks are commonly used for storing water for various purposes, such as domestic use, agriculture, and industrial applications. However, water tanks can become a breeding ground for fungi and other microorganisms, leading to contamination of the stored water. To prevent the growth and spread of fungi in water tanks, various chemicals can be used. One such chemical is copper sulfate.

**Copper Sulfate:**
Copper sulfate, also known as cupric sulfate, is a chemical compound that contains copper, sulfur, and oxygen. It is commonly used as an agricultural fungicide and algaecide due to its ability to control various types of fungi and algae. Copper sulfate is highly soluble in water, which makes it effective for treating water tanks.

**Mechanism of Action:**
Copper sulfate acts by disrupting the cellular processes of fungi. When added to water, copper sulfate dissociates into copper ions (Cu2+) and sulfate ions (SO4^2-). The copper ions are toxic to fungi as they interfere with the enzymatic reactions necessary for their growth and reproduction. Copper ions also disrupt the cell membrane of fungi, leading to leakage of cellular contents and ultimately cell death. Additionally, copper sulfate inhibits the synthesis of proteins and enzymes in fungi, further suppressing their growth.

**Advantages of Copper Sulfate:**
- Effectiveness: Copper sulfate is highly effective in controlling a wide range of fungi, including those commonly found in water tanks.
- Residual Activity: Copper sulfate has residual activity, which means it continues to provide protection against fungi even after application. This makes it suitable for long-term control of fungal growth in water tanks.
- Cost-Effectiveness: Copper sulfate is relatively inexpensive compared to other fungicidal chemicals, making it a cost-effective option for treating water tanks.

**Conclusion:**
In conclusion, the chemical used for destroying fungi in water tanks is copper sulfate. It acts by disrupting the cellular processes of fungi, leading to their death. Copper sulfate is effective, has residual activity, and is cost-effective, making it a suitable choice for controlling fungal growth in water tanks.