As has been said before, the tank is entirely too small for a Goldfish
; I've had some Goldfish
that would not physically fit in a tank that size. They easily grow to 6+ inches. Three is not old age as they regularly live to 10 or even 20 years old when given proper housing. There are a lot of myths out there about Goldfish
; their short lives and small size are two of the biggest.
Your fish is having swim bladder problems. These are quite common with fancy Goldfish
kept in poor conditions, but I don't blame you for it as most pet stores just don't understand Goldfish
care. Feeding a pea will not help as it is not a digestive issue.
That being said, the only way to help your fish is to get her a larger tank. Since I see she's a common Goldfish
(doesn't have that chubby body and double tail), she will need a 40+ gal tank. She still has some growing left to do. And with clean water and some good food she could live close to a normal life.