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In the sentence "Many vivid tropical fish swam beneath our glass-bottomed boat," there is no need for any commas. The sentence is correct as it is.
Here's why:
1. The sentence consists of a simple subject ("Many vivid tropical fish") and a predicate ("swam beneath our glass-bottomed boat") without any clauses that require commas for separation.
2. Commas are typically used to separate items in a list, set off introductory phrases, indicate a pause, or separate clauses in a sentence. Since none of these situations apply in the given sentence, it does not need any commas.
Therefore, the sentence is correct as it is written.
