You mentioned that the fish is "swimming upside down in circles". Whilst the fish is swimming in this manner is it "diving" up and down or is it swimming in circles in a horizontal manner?
Thanks for the information.
Your fish is suffering from something called Whirling Disease which is caused by a parasite that invades the fish's head and spinal cartilage and is what's causing the fish to swim in circles. Sadly there is no known cure for this ailment and it's usually recommended that an affected fish be euthanized.
Some fishkeepers say there is a link between feeding fish Tubifex worms and contracting Whirling Disease but I have had a fish and know of others who've had fish
with Whirling Disease and Tubifex worms were not part of their diet so the link is still considered speculative.
Listed below is a link which will describe a very humane method to euthanize fish.
(Read area under-"Preferred Method: Clove Oil and Tank Water Method")
I wish there was a better prognosis and please keep in mind that this illness did not occur because of anything you did. It just randomly occurs.
The Tubifex worm theory is exactly that--a theory. I personally believe the Tubiflex-Whirling disease link is just a coincidence.
Based on my experience Whirling Disease is not contagious so there's no need to be overly concerned about the other fish catching it.
Your situation reminded me of my first encounter with Whirling Disease. I was a beginner and it happened to a Black Molly. It took me a while to finally find out what the diagnosis was. Thank goodness this horrible ailment is rare.
Take care Nicole and thank you for your accept.