There is a YouTuber called life with pets (I think!) who had a similar problem with one of her goldish. She ended up putting it in a shallower, smaller tank and it helped some. There may be more videos on the channel! Aqua Girl Talks also has some videos. Have you ever tried Repashy Super Gold? I was so tempted to buy a goldfish online, but I ended up at my local LFS and bought a derpy ranchu after watching all the fish for an hour. So disappointing they didn't send you the video! If I ever buy online, it with be with Zhao's Fancies. More expensive, but they all have videos