Fish are like any other animal they have their own personalities, you can get two fish out of the same tank in the LFS and get them home and you find out they don't like each other any more. You can also get two fish that are the same breed to find out one is goo with some fish and the other one is not. You can also have some fish and when juvinile they are fine but when they become adults they change and show their real colors litraly. Alot of eels that are labled not reef fendly sre quite compatible just when they get older they get large and start knocking corals over and stuff once a coral is turned over they trigers and tangs and some angles think it feeding time. So unless you get all fish that you are going to keep in the tank as juviniles or buy fish of euile size as to what you already have, it becomes trial and ERROR.

Bill