Which one is correct in American English?

The most natural way to say this is, "Imagine giving a fish more food than it needs" or "Imagine a fish getting more food than it needs."

I think it's interesting that you used "she." Is "fish" a feminine noun in your language? In English, an animal whose *** is unknown is usually referred to as "it" or "he". Pets, of course, are called "he" or "she" according to their actual ***. (

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So, are all of my sentences correct?

bassaa)Imagine giving food to a fish that is more than what she needs. Do we need that "that " after fish?
b)Imagine a fish being given food more that is more that what she needs. Do we need that "that " before is?
c)Imagine a fish being given more food than what she needs.

Your sentences are all awkward in English, for various reasons.

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Why is it incorrect to put what in (a) and (c)?

The "what" is not really incorrect -- it's just not necessary. You can keep if it you want to.

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Yes, fish in my language is feminine and cats,the sun,flowers,ducks,snakes,chikens....... .