Dave Nitsche was a healthy triathlete — until a bizarre vision problem in 2019 stopped him in his tracks. “I would look at telephone poles … they seemed kind of wavy,” Nitsche, a father of two and grandfather of one, told The Post. He went to an optometrist, who suspected a detached retina and urged Nitsche to go to the hospital. Fluid had built up behind his left eye, and the eye was removed two days before his 50th birthday. The fluid turned out to be cancerous. Then, over a liter of fluid