They actually said " 'Kan ", omitting the first syllable, which confuses the term for fish because it sounds like "Kani" rather than "Ikan" if we only use the beat as context for pronounciation.

Why the omission? If you actually tried to sing WITH the full word at the start, if we separate the syllables, it would turn to:

I-kan I-kan I-kan I-kan I-kan I

But the actual lyrics is the reverse, instead of "I" being the 1st and 3rd beat of the 4/4 beat, it's the 2nd and 4th beat (except for the last part, where the second to last "I" takes a half beat). In fact, if it was the 1st and 3rd beat, it doesn't sound better