Maybe bullying from the new ones? I'm not very familiar with goldfish. Is he eating still? Appear sick? How do they spawn?
Sounds sick. Do you have a hospital or quarantine tank or some place to move it to?
I'm sorry. Alot of times stress will weaken their immune system and leave them susceptible to bacterial or fungal infections. Alot of the problems don't manifest themselves visibly until it's too late. You could try some food laced with antibiotics
There's an easy recipe for making it. Aquarium science. Com, Aquarium co-op and Hikari all have a recipe for making it. The one I make uses plain unflavored gelatin, hot water, flake food and antibiotic. Mix it into a paste and refrigerate it to let it set. Then cut it into very small pieces and feed once a day for 7 days. You can do the same thing with general cure as a dewormer for 4 days. It'll keep in the fridge for 2 weeks, or freezer indefinitely