Should I euthanize my tetra?

I have a 30 gallon with two d. gourami and one angelfish and 3 harlie rasboras. I also have two silvertipped tetras but one of them is swimming lethargically in the corner, while the other one plays and appears to be fine. how long should I wait to dispose of him. i'm worried he'll come down or has come down with a disease that will spread to the other fish which are healthy and happy.