Hi,. I have 15 gallon tank. The aquarium shop guy told possible to put goldfish with fighter. It will don't fight with goldfish or guppies. It calm in the edge of tank. But i fear to put same tank both. What can i do?? Now i have 1 fighter. Is it possible or not? Tell me please,. Thank u.

Aquarium people know well than others. Follow them.

goldfish are coldwater fish, bettas are tropical they cant live together. some bettas can live with guppies, but its not advisable. and 15 gallons is not big enough for a goldfish

A Goldfish would not only eat a Betta (also known as a "fighter) but would also grow out of a 15g tank. Goldfish get huge, are very messy, and have the opposite water temperature and diet requirements to a Betta. A Goldfish would also eat Guppies.

Bettas and Guppies are hit and miss, mostly miss as male Bettas tend to mistake male Guppies as other Bettas and kill them.

If the Betta is already established in a 15g you have a problem. To keep Bettas with other fish the Betta should be the LAST fish in the tank, otherwise all newcomers could be viewed as a potential threat to his established territory. They can work with bottom dwellers like Amano Shrimp and Corydoras.