Von Rio Tetra with Red Sore

[FoundMoney]

I have one von rio tetra in my hospital tank with a swollen red spot under its skin, on the side, behind the gill. The sore developed about 3 weeks ago and I isolated the fish immediately. No other fish have shown the same symptoms. I treated the tank with Quick Cure for parasites, Maracyn-2 and erythromycin, yet the swollen red sore is still there. I'm not sure what I should do besides just leaving the fish in the hospital tank. However, if the fish is sick then it makes it hard to use the tank for quarantining new fish or other sick fish, without risking this fish will infect them with whatever it has. Any suggestions?

 

[The-Wolf]

Hmm given the time frame it sounds like he has either a
parascitic cyst or a tumor.

Cyst
this should be cleared up with a strong anti-parsite medication like malachite green and formalin (used together)
see here for how http/www.fishdoc.co.uk/treatments/malachite.htm

Tumor
Tumors can be caused by a virus or a cancer, but most tumors are genetic. The genetic tumors may be caused from too much hybridization, common amongst professional breeders.
Practically all tumors are untreatable. If the fish is in distress, it should be euthinised.

HTH