Sorry, I don't mean to sound blunt but you need to be doing weekly water changes of at least 50 percent, that's just regular maintenance for a well stocked tank, most experienced members on here do fin level water changes weekly.
feeding live as a staple shouldn't be goldfish as they are fatty if that's what your doing, the risk of disease is massive buying feeders, you should breed your own, convicts or molly's would be better.
Not sure if it's the metro in the product I said but it's the only med I'm aware of that works, waterlife Octozin..
I have no doubt that your wc schedule and possibly diet has caused this pal.
Try to get him on frozen fish.
Read up on here via the search bar about nitrate creep and water changes.
Goodluck. Justin.