Although I'm far from an expert, I have had my 3 Fantail Goldfish for years now.
They have a 220 litre tank and up until now have been healthy.
One of my fish (Bowie) has been unable to stay afloat for any long period of time, sinking to the bottom each time he swims upright. His belly looks bloated. I fasted him for 3 days, no effect. Then I have been feeding him peas, still no effect. I have now been using the Epsom salt bath routine and although this allows him to do a big fish poop, he still has the problem of sinking to the bottom.
Any help would be great, he looks really miserable. Still eating well.
Up until this week I would give him Bloodworm once a day, as well as flakes. I've since changed over to the sinking pellets.
Water quality is good too.