Welcome to the forum, Chul.
Please tell us the name and author of the book you are studying from and the topic of which your quoted sentences are examples.
1. The more common plural of 'fish' is also 'fish', like 'deer' is both singular and plural, so 'Even fish sleep at night' is correct.
'Even fishes sleep at night' is also correct, but is less common.
To make it singular, you need 'Even a fish sleeps at night'.
2. I agree with you.
Note that 'This sentence is correct?' should be 'Is this sentence correct?'
Interested in Language