Fish sauce, like olive oil, has different grades, which determine its quality. Good fish sauce is the color of weak tea and smells like the sea. I have seen dark-colored fish sauce, which looks like soy sauce, in the best markets. The brand does not guarantee the quality of the fish sauce. It is the color that tells it all.
Fish sauce has a short shelve life.
If you are not going to use fish sauce everyday, buy the smallest bottle you can find. I prefer a glass container, fish sauce seems to stay unspoiled longer in glass than plastic. Once open, fish sauce should be refrigerated. When it becomes dark, discard and buy a new bottle. Buy a couple of brands and experiment.