I have a fancy goldfish that I've had for almost two years now. For the past week she's been acting really strange. I found her floating upside down at the top of the tank and assumed she was dying, but a few hours later she was swimming around perfectly fine as if nothing at all had happened. Now she is floating upside down---but at the *bottom* of the tank.

I've tried to research swim bladder disease but it doesn't really seem like it's what she has. She does swim normally, she sometimes is floating upside-down for extended periods of time. She always seems fine afterwards.

I haven't changed anything at all with her routine except add a live plant and change her water frequently.

What's wrong with my fish? If it is swim bladder, how can I cure it?

Thank you in advance!