Goldfish need 10 to 20 gallons per fish, and will grow large enough to eat the betta. Also I have had a few times when the power went out and my betta had to go into my 125 gallon goldfish tank to stay warm enough. The betta went after my 12 inch fancy goldfish and ended up staying in a breeder net til the power came back on. If you want goldfish you would need 20 gallons for the first fish and 10 for each additional fish. Commons, comets, shubunin and walkin need 20 gallons per fish.