One of my fish had popeye just a few weeks ago after I had to rearrange the tank to catch an unwanted fish. I cured it with just a large water change and the use of garlic.
If the popeye is caused by trauma alone, it will not kill the fish. However, it can lead to permanent blindness in that eye. If the popeye is caused by poor water quality, than other problems will often set in and can kill the fish.
A new tank will often have another minicycle (with another spike in ammonia and/or nitrite levels) whenever a new fish is added. That is why it is recommended to add fish very slowly and let the bacteria that were started in the cycle to keep up with the added wastes associated with more fish. Use the time to quarantine any new fish for ~3-4 weeks.