Tropical freshwater, what is fish compatibility exactly ?

there is no exact science to say what will work together and what wont it is a basic guide for general use some people have a mix of fish that dont go together. some fish that dont mach some times works together for a while some dont it pending on fish to fish i knew i guy who set his tank up with fish that didnt mix they worked together the second tank with the same type of fish but a bigger tank they killed each other it is a general rule that apply to fish then their is some fish that dont mix period