About a month ago, one of the two goldfish in my 55gal, an oranda, suddenly passed away. There was nothing visibly wrong, no signs of stress at all and the water params were perfect. We were sad but figured it must be just a freak occurance. Well, now the other, a ryukin/pearlscale hybrid is upside-down. Other than being upside-down, there seems to be nothing wrong. He's a male and I can see some breeding tubercles out, but I think it all looks normal. I've seen dropsy, and this is not dropsy. I know it's a long-shot, but any ideas? I'm just desperate. The only thing I can think to do is try to feed him some peas maybe if he'll eat, but it's not looking good. Does anyone have any advice for me? The other inhabitants of the tank are some cories and bn plecos but they all seem fine. What the heck is going on with my poor goldfish?! Thanks.