I think what Kelly meant by poisoning was from the fish themselves, when fish produce waste (ammonia) this converts to nitrite then nitrite to nitrate, both ammonia and nitrite are very harmful to the fish and nitrate in high levels are also harmful to fish.
When stocking a tank it is recommended that you stock per their adult size not current size as the plan is that the fish grow up and you keep them for their natural life span, also, stock consideration of swimming habits, territory needs, and social needs. Goldfish
can live over 20 years with proper care and housing, and the common type as stated can get really big and thrive best in ponds, they also thrive best is cold water.
IMO it sound like your fish are suffering from poor water quality, overstocking, poorly stocking issues.
I would recommend that you rehome the common or singletail Goldfish
and get a bigger tank for the fancys, until then.... check your nitrate level and if it is less than 30ppm start making 50% daily water changes for 7-10days then stay on a regular schedule of weekly 50% water changes and substrate vacuuming, hold food for 4-5 days, get the water temp in the 50-60F area, test your water daily until it is stable with 0ppm ammonia/nitrite and nitrate 5-10ppm