YOU SAID:
Theirs a lot of fish in the sea
INTO JAPANESE
彼らの海にはたくさんの魚がいます
BACK INTO ENGLISH
there are many fish in their sea
INTO JAPANESE
彼らの海にはたくさんの魚がいます
BACK INTO ENGLISH
there are many fish in their sea
That didn't even make that much sense in English.