Bettas prefer warm temperatures such as 78-80 degrees, and gold fish are cool water fish and prefers 55-70 degrees this makes them very incompatible as tank mates.
The male Betta will chase any kind of long fins because they see it as another male betta.( I assume it's a male betta)

Two male bettas in the same tank will fight usually until one is very beat up or dead.
The bettas need at least 5 gallons of water each. and the goldfish needs at least 10 Gallons preferably 20 Gallons because they produce so much waste. Hope that helps.
Carol
By the was WELCOME TO FISHLORE!!!
