1. Size difference: Koi can grow much larger than red fantail goldfish, sometimes reaching lengths of over 3 feet. This size difference can lead to competition for food and resources, and the koi may accidentally injure or even eat the smaller fantails.
2. Water temperature: Koi prefer cooler water temperatures, typically between 65-75°F, while red fantail goldfish are more adaptable and can tolerate a wider range of temperatures, including warmer waters. Keeping them together may require compromising on the ideal water temperature, which could stress both species.
3. Diet: Koi and fantails have different dietary requirements. Koi are omnivorous and require a varied diet that includes plant matter, insects, and small fish, while red fantail goldfish are primarily herbivorous and thrive on a diet of high-quality fish food and plant matter. Mixing them together may make it difficult to provide each species with the appropriate nutrition.
4. Disease transmission: Koi and goldfish can carry different diseases and parasites that can be transmitted between them. Keeping them together increases the risk of disease outbreaks and can make it challenging to treat and manage any health problems that arise.
5. Aggression: Koi are known to be more aggressive than red fantail goldfish. While fantails are generally peaceful fish, they can be bullied and harassed by larger, more aggressive fish like koi. This can lead to stress, injuries, and even death for the fantails.
Therefore, it is generally best to keep koi and red fantail goldfish in separate ponds or aquariums to ensure their well-being, health, and compatibility.