Join Date: Jun 2011
Location: Orlando Florida
Goldfish and Betta's are not good tank mates for a few reasons.
The Betta will nip the fins, and they need different temps. With Betta's being around 80* and Goldies being low to mid 70*.
And I am also recommending you do not use the bowl. It is not able to be filtered, which all fish need, and Goldfish are very messy. They should have double filtration. If Betta's are not OK in bowls, I doubt that Goldies will be. You will stunt them, and they will be goners.
The smallest Goldfish also gets 6 inches, that is the fancy goldfish, and need 30 gallons as an adult. Koi's get over 2 feet and should be strictly pond fish. Some people keep them in large tanks, but I always like Koi in a pond.