size, color, health, behavior, feeding/not skinny. It all plays a role. Its going to be your new fish so it needs to be a size you can handle and that will go with your other fish. Color has to appeal to you. Behavior has to be normal and the health has to be perfect. If it isnt acting right or you are unsure about its health, it is not worth the risk of brining home a diseased fish that you now have to cure. A fish that is stressed and not eating already has issues. If you cant fix those problems quickly, a fish in poor condition may only die. Pick a strong healthy fish that you love the look of