raise the temperature of your tank....if as severe as you hint at...I'd go all the way up to 86 degrees. Keep the tanl light off and make it ambient light when needed
Feeding peas is good.
Some Marycyn 2 would help as well. I would use some anti-biotic food if possible. Antibiotic food works so much better.
If you don't have any medi-gold or metro-gold or similar. It should be found at some pet supply, or online at like goldfish .come/.net or dr foster's& smith
Also about 1 tablespoon/5 gallons of EPSOM SALT. To relieve the worse symptoms....
and as soon as the symptoms abate....keep treatin but see if it will eat better...possibly give it a high caloric like nutrical if it seems t lose too much weight during the ordeal. if the scales are not straight out yet the chance of recovery are not bad....
"Fish! Thank You! Oh, tropical huh? Did you buy a heater? Auuugh!"
Mega-pet stores prolly should not be allowed to sell animals as retail "items".