Complex sentence: By the late-1950s, Tony Tashnick had firmly established himself as one of the best all-around swimmers in the United States; setting US Open records in the 100 and 200-yard butterfly, and 200-yard individual medley.
Split into simpler sentences: By the late-1950s, Tony Tashnick had firmly established himself as one of the best all-around swimmers in the United States. Tashnick set US Open records in the 100 and 200-yard butterfly, and 200-yard individual medley.