Goldfish are a cold water fish. They can take quite a variety of water conditions. They prefer cooler water and can tolerate a 100% water change if kept in small bowls. That is usually a pain to do, so most people either keep them in a pond or a small aquarium (10 to 29 gallons) with a small MOB filter. You do NOT need a heater for these fish as they prefer water temps less than 72 degrees and can tolerate wtare tempertures in the 40 degree range. Feed sparingly as they poop a lot and the more you feed the more they poop and the more often you will need to do water changes. Feed a small amount twice a day and make sure none of the food goes uneaten. If it does, then you are feeding too much or too quickly. If you have any other questions about freshwater fish, or goldfish in particular, please do not hesitate to call on me.