Is it still pooping?
Mine were like that, well one was. I tried everyting, peas, epsom salts, metronidazole, praziquentel, flake levimsole, nothing ever worked.
I would suggest to you to stop feeding entirely for a couple days, and when you do resume feeding, only 2 small meals a day of nothing but veggie (spirulina, peas, algae flakes or wafers for plecos)
And remember to always soak goldfish pellets in a small jar for 5 minutes and then dump them into the tank, goldfish have weak stomachs and tend to gorge themselves.