Article: Remember, you need 10 to 20 gallons of water per fish. If you add a new fantail goldfish to your tank, you will need to increase the tank size accordingly. If your fish feel overcrowded, they may become aggressive and fight with one another. Even in a properly sized tank, some goldfish are simply more territorial than others. If one fish seems to be consistently chasing another, invest in a tank separator to manage conflict.  You can buy a tank divider at a pet store. Install it in your tank so your fish cannot get to one another. You can also buy a new tank altogether to correct this problem. Uneaten food at the bottom of the tank can be a problem. Not only does this indicate you're overfeeding your fish, this could cause tank contamination. If you notice un-eaten food during a routine water change, remove it from the bottom of the tank. In the future, cut back on how much you're feeding your fish. Fantail fish can survive in a variety of temperatures. However, sharp increases or decreases in tank temperature can  shock your fish's system. Take steps to assure the tank temperature remains relatively stable.  Do not place a goldfish tank near a window. The air from outside can cause the tank temperature to rise or drop quickly. Keep the tank in an area of your home where the temperature is stable. Watch for areas of the home prone to drafts, or that get warmer or colder during certain times of the day. You should also quarantine any fish that appear unhealthy. This not only prevents the spread of disease, but allows you to closely monitor the sick fish and treat the illness without harming the other fish, plants, and invertebrates in your tank with medicines and chemicals. Signs of illness can include:  Bloated body Listlessness White spots on the body Rapid breathing Bulging eyes Hiding in the corner

What is a summary?
Avoid overcrowding. Consider a separator if the fish become territorial. Watch for uneaten food at the bottom of the tank. Keep the tank temperature stable. Watch for signs of illness.