The lifespan of fantail goldfish can be expected to be about 10-25 years. Unfortunately, few fish actually make it past the three year mark because of their owner's inexperience. Here is a list of things you can do to improve your fish's chances of living a very long and happy life: buy an aquarium of at least 30 gallons (more preferably), feed them a high-quality fish food and give them plenty of variety in foodstuffs, do weekly 50% water changes, and observe your fish each day for about 15 minutes to look for any signs of illness.