Read the following statements carefully and find the incorrect statements:
A. More than 1 percent impurities make domestic sewage unfit for human use.
B. Less BOD causes mortality of fish and other aquatic creatures.
C. Presence of large amounts of nutrients in water also causes excessive growth of planktonic algae, called an algal bloom.
D. Algai bloom causes deterioration of the water quality an decreases fish mortality.
E. Some bloom-forming algae are extremely toxic to human beings and animals.
F. Eichhornia crassipies is the world's most problematic aquatic weed.

Options

A. A, C , E and F
B. C , E and F
C. A, B and D
D. All are correct

Answer

To solve the question, we need to analyze each statement carefully to identify which ones are incorrect.

### Step-by-Step Solution:

1. **Statement A**: "More than 1 percent impurities make domestic sewage unfit for human use."
- **Analysis**: This statement is incorrect. Domestic sewage becomes unfit for human use with just 0.1% impurities, not 1%. Therefore, this statement is false.

2. **Statement B**: "Less BOD causes mortality of fish and other aquatic creatures."
- **Analysis**: This statement is also incorrect. A high Biological Oxygen Demand (BOD) indicates a high level of organic waste in the water, which can lead to oxygen depletion and subsequently cause the mortality of fish and other aquatic creatures. Thus, less BOD does not cause mortality; it is the high BOD that does. Therefore, this statement is false.

3. **Statement C**: "Presence of large amounts of nutrients in water also causes excessive growth of planktonic algae, called an algal bloom."
- **Analysis**: This statement is correct. High nutrient levels in water bodies lead to algal blooms due to excessive growth of algae. Therefore, this statement is true.

4. **Statement D**: "Algal bloom causes deterioration of the water quality and decreases fish mortality."
- **Analysis**: This statement is incorrect. Algal blooms do cause deterioration of water quality, but they increase fish mortality due to oxygen depletion and toxins produced by some algae. Therefore, this statement is false.

5. **Statement E**: "Some bloom-forming algae are extremely toxic to human beings and animals."
- **Analysis**: This statement is correct. Certain types of algae that form blooms can produce toxins harmful to humans and animals. Therefore, this statement is true.

6. **Statement F**: "Eichhornia crassipes is the world's most problematic aquatic weed."
- **Analysis**: This statement is correct. Eichhornia crassipes, commonly known as water hyacinth, is indeed considered one of the most problematic aquatic weeds globally. Therefore, this statement is true.

### Conclusion:
The incorrect statements are A, B, and D.

### Final Answer:
The incorrect statements are **A, B, and D**.