Chuck Palahniuk. Survivors (№ 409)

To calm this girl down, to get her to listen, I tell her the story about my fish. This is fish number six hundred and forty-one in my entire life. My parents bought me the first one to teach me how to love and care for another living breathing creature of God. Six hundred and forty fish later, the only thing I know is everything you love will die. When you first meet someone special, you can be sure that one day he will die and will be in the ground.
Another option:
... This goldfish, and I have not the first. It is six hundred forty-one. They are very long live, Golden fish. The first fish I bought parents that I learned to love and care for the living creatures of God. And that's all I know six hundred and forty fish later: all that you love will die. And when you meet someone special, you can be sure that one day it will not. He will die and turn to dust.