Although goldfish are considered to be the ideal fish for beginners, this is actually not the case. Goldfish are coolwater fish that need lots of space and high concentrations of dissolved oxygen in their water. Compared to many types of tropical fish, goldfish are large fish, with greater body size for their length. Maintaining good water quality for goldfish means dealing with the increased amount of waste products these larger fish produce, and making sure the water is well-aerated and not too warm, so they have sufficient oxygen in the water. Fancy goldfish have the same requirements, but should not be kept in water cooler than 60 degrees Fahrenheit. To avoid overstocking a goldfish aquarium, there should be approximately 20 gallons of water, or more, for each adult fish - regular goldfish can grow to almost a foot, and fancy goldfish can attain a length of 6 inches and more. Goldfish need a lot of vegetable matter in their diet - cooked, slightly mashed peas are an excellent supplement to high-quality goldfish foods.