The other main component to avoiding disaster when adding a fish to your aquarium is to make sure that it is healthy. This is done at the fish store, not at home. When you decide on a fish, take a few minutes to look at its behavior in the fish store aquarium. Some fish by nature tend to sit at the bottom, but if the species you are looking at is not as active as it should be, this is a good indication that it is not in good health. Also, don't be afraid to ask the store clerk to feed the fish.
Healthy fish are almost always willing to eat.
If the fish seems uninterested in the food, it may also be an indication that something is not right. The last major thing you can do is to take a look at the physical condition of the fish. These are important questions to ask yourself before you take a fish home. By trying to make sure that the fish is in good health, and by making the transition from the store to your aquarium less stressful, you will decrease the chances that your new purchase will lead to disaster. There is no way to completely eliminate all the potential dangers, but this will allow you to have a better chance at success.