YOU SAID:

the fish ate the other fish and then preceded to swim away with the other fish still in its belly

INTO JAPANESE

その魚は他の魚を食べてから、その腹に残ったまま他の魚と一緒に泳ぎ始めた

BACK INTO ENGLISH

After eating the other fish, the fish left swimming in his belly and started swimming with other fish

INTO JAPANESE

他の魚を食べた後、その魚は彼の腹に泳ぎを残し、他の魚と泳ぎ始めました

BACK INTO ENGLISH

After eating other fish, the fish left a swim in his belly and began swimming with other fish

INTO JAPANESE

他の魚を食べた後、魚は彼の腹に泳ぎを残し、他の魚と泳ぎ始めました

BACK INTO ENGLISH

After eating other fish, the fish left a swim in his belly and began swimming with other fish