There are many reasons why these two fish cannot be tank mates. First of all a betta fish is a tropical fish, originating in the waters of Thailand, the betta needs waters of 78 – 80 degrees Fahrenheit. Goldfish however prefer cooler waters of around 74 degree Fahrenheit.
Can a betta fish and goldfish live together?
Goldfish produce a lot of fish waste, leading to high levels of ammonia in the water which is toxic to betta fish. It dirties the water in the tank quickly, meaning that larger more frequent water changes need to be made. Large and frequent water changes stress betta fish (although they are better than leaving your fish in dirty water!).
The number one rule when you think about what fish can live with your betta fish is no fin nippers! Guess what… Goldfish can be bad fin nippers! That does not go well with the long delicate fins of a betta – that or the betta’s short temper!
Another difference is their diets. Betta fish are carnivores – living off small insect larvae in the wild. Goldfish however, are omnivores meaning that they require both vegetation and protein. Betta fish specific food is not recommended for goldfish.
So, I guess that is reason enough to suggest that you never partner a goldfish and a betta fish in the same tank.
Hope this has helped!