Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If the fish is oddly shaped, distorted, bent spine or other issue I would euthanize and sterilize the tank.
If the fish is just thin from poor eating then treatment is an option. I would start with antibiotics, as red under the skin can be one sign of an internal bacterial infection.
Never put fish in the main tank without quarantining and treating them. Minimum time in the quarantine tank is 1 month, and that is only if no treatment is needed.
This gives you plenty of time to see that the fish are healthy, or to start treatment as needed. This way also you are not exposing your main tank to diseases or parasites, or the treatments for these problems. You can customize the medication to suit the fish and the disease without worrying about killing other livestock or plants.