Just the symptoms sound like some sort of poison/neurological issue, being an aussie fish that does make it slightly less likely. it also almost sounds like the disease that fairy wrasses will get.... but that's just for wrasses. Might want to get the opinions of the #fishmedic Group though
That looks intense. #fishmedic
Hopefully the experts will come by to help. I would try to take him out of the tank and put him in at least observational quarantine and be ready to treat with a broad spectrum antibiotic.
Crossing fingers that he turns around soon.
...spread to anything else in your tank.
That said, let's get some disease-expert opinions here on if that's the problem your fish have: #fishmedic
For future reference, I'd suggest reading through the link below (particularly the "Aquarium Parameters," "Water quality," and "In-depth...
#fishmedic
@Jay Hemdal and @vetteguy53081 looks like this cardinal needs your expertise.
I have never seen this before. Hopefully he gets better soon. Hopefully the guys that know a lot about fish disease and treatment will reply soon! Good luck!
Looks somewhat like ich, but don't take my word for it.
If you have a cleaner shrimp in the tank, you shouldn't treat the entire tank, or you can take the cleaner shrimp out.
A #fishmedic will help you out soon.