You have to be careful picking out a new fish for your marine aquarium. The last thing you want to happen is to take your new home and it becomes sick due to a disease or something else that you could have identified at the store. There are a few things to watch for that will give you an indication there could potentially be a problem with a fish. These indicators to watch for are both in the appearance of the fish as well as how the fish behaves.
Selecting a healthy fish is not a hard thing to do but it does require patience and restraint. If you are not 100% sure that a fish is healthy , it would likely be best not to buy it. The below are the basic rules of thumb that I follow. Keep in mind, following a process like this can never be 100% safe or 100% accurate, so you should still quarantine new additions before add them to your display tank.