YOU SAID:

There's a fish swimming in my glass of cola.

INTO JAPANESE

私のコーラには魚が泳いでいます。

BACK INTO ENGLISH

Fish are swimming in my cola.

INTO JAPANESE

魚は私のコーラで泳いでいます。

BACK INTO ENGLISH

The fish is swimming in my cola.

INTO JAPANESE

魚は私のコーラの中を泳いでいます。

BACK INTO ENGLISH

The fish is swimming in my cola.