I cannot agree more with what cowboy said. You have to follow your heart. There was a yellow double tail at my local PetSmart for two months that was just torturing me. I finally ended up buying him as a rescue because he had gotten sick and his fins were torn.
He lived for a week in my care, and his fins even started to heal. In the end though, he didn't survive. Point is, if your gut is telling you to get that fish, do it! Don't wait! I sometimes wonder if Agrippa (that was his name) would still be alive if I had rescued him sooner