Complex sentence: Evans was an All-American for four years in the collegiate system, but he struggled in the short-course pools used in the college system, which placed more emphasis on a swimmer's turning ability, which was Evans' weakness.
Split into simpler sentences: Evans was an All-American for four years, but he struggled in the short-course pools used in the college system. Short-course pools place more emphasis on a swimmer's turning ability, which was Evans' weakness.