Pick a fish that you definitely want that is compatible with the size of the tank you are looking at and work around that. For instance, if you want a lionfish, you now know that you have to avoid most triggers and all fish that are generally smaller because larger lions will eat small fish.
Plus, it does not help any when you post in one thread you are getting a 180 and in the other you are looking at a 240. What size are you really getting?