Original: it is Eight fish that the pelican caught.
Best corrected sentence: The pelican caught eight fish.
Why this is better (simple steps):
- Start with a capital letter: "The" not "the".
- Put the subject first: who did it? The pelican. Then say what happened: caught eight fish.
- Use a period at the end of the sentence.
- Write the number as a word when you are writing a short sentence: "eight" (or you can write the digit 8).
- Use "fish" for the plural of fish (we usually say "fish," not "fishes").
Another correct but less natural version: It is eight fish that the pelican caught.
But the shorter sentence "The pelican caught eight fish." sounds clearer and is easier to read.