I agree with lordcheng..regular maintenance of the water and diet of the fishes are one of the basics in keeping them healthy and happy. Providing them more hiding places make them comfortable and live longer.
I have 16 goldfish in different types and my tank is not big enough. I do my regular maintenance and my fishes are so happy. In times of feeding they gather with a dance around my hand. I am happy with my pets!

hehehe...i have 15 small gold fishes in 10 gallons in my aquarium but they live in a very healthy life its because of my secret technique in maintaining the water and feeding them..although they have very small area but they live in a happy and healthy life..

This really is NOT true. For one fancy goldfish it should have about 15-20 gallons (20 is better) and then an additional 10 gallons for every additional fish :D If it's a smaller fish though, a 10 gallon for 2 then an additional 5-10 for every new (small fish you add). So a 10 gallon would best hold 2-3 (small goldfish) or 1 medium goldfish with 1 or 2 small ones. No fish bigger than that though. I think that is pushing it a little though. I have a 20 gallon with 2 baby Ryukin and I hope they don't get more than 6" long!!!

I wish I knew that when I was seven!