Goldfish trouble need advice please?

Goldfish showing various symptoms and nothing seems to help. I adopted a 4yr old white goldfish wih a bent tail. He seemed to thrive until we moved him and his brother to a 50l tank where he gradually started showing signs of not eating. At the moment he is in quarantine with a completely curved spine, floating on his side at the surface and struggling to swim. I've tried feeding him peas and other food but he spits it out everytime I manage to get anything in (a lengthy process with a chopstick). He's also trailing a long thin white string of what I imagine is poo. I've medicated the quarantine water and added aquarium salt. I can't let the poor fellow die! Desperate to know what's wrong. Please help my little guy.