I honestly think it's to late ,sorry.Take it to the nearest animal hospital in a large cooler as soon as possible. I'm not sure what that could be, but it is best to find a professional as soon as you can.
if that's what it is then they would have had to add a substantially higher temperature water to the aquarium quick fast and in a hurry.It's gas bubble disease. I couldn't find anything to treat it, especially at this stage. ALL other fish you have could be in danger.