Compatibility

• When selecting fish for a fish tank, it is important to determine whether the species are compatible. Aquarium fish are described as aggressive, semi-aggressive or community fish. Both guppies and goldfish are community fish that do well in groups of their own species and with other peaceful species of fish. Though they do not show aggression toward each other, there are other factors that make goldfish and guppies incompatible tank mates.
  
  

Fish Size

• When determining whether it is safe to mix certain species of fish, consider the potential size of both species. In general, large fish tend to eat smaller fish. Guppies only grow to a maximum size of 2 inches and goldfish grow to be quite large. With this being true, large goldfish are almost expected to eat guppies if they are placed in a tank together. There are other factors besides size that are also taken into consideration.
  
  

Tank Requirements

• Guppies are known for being hardy fish that are great for beginner tanks. Guppies are capable of tolerating varying water temperatures and do not require a great deal of space. Goldfish, on the other hand, require at least 20 gallons of water volume and excellent filtration because they have a high waste output. The waste output of goldfish tends to lead to a quick buildup of ammonia and nitrates, which is harmful to guppies if they are kept in the same tank.
  
  

Water Temperature

• Aquarium fish are generally divided into two different groups by the type of environment in which they live, saltwater and freshwater. These groups are further divided by water temperature, tropical or cold water. Goldfish and koi are cold water fish, which makes them ideal for use in outdoor ponds. Guppies, though able to survive in cooler temperatures, are tropical fish and thrive best in water between 75 and 80 degrees Fahrenheit. Because of these differences, goldfish and guppies are not to be kept together.