For optimal health of your fish it is important to avoid overcrowding. Below is a general guide for number of fish optimal in different size goldfish tanks.
Use this guide when choosing the number of goldfish and/or size of the tank you are purchasing when setting up your personal tank.
1 Gallon = 1 Goldfish
5 Gallons = 2-3 Goldfish
10 Gallons = 3-4 Goldfish
20 Gallons = 6-8 Goldfish
30 Gallons = 9-12 Goldfish
Goldfish in general are a hardy fish. They are generally active and are capable of significant growth. It is important to avoid overcrowding your goldfish tank as it may result in bad water conditions. The more space per goldfish the healthier your fish will be.