"Agressive" is a relative term with fish. Some fish hate only the same kind. Others hate different fish. Some hate everybody. Knowledge of the species is crucial in selection. You must keep in mind that not all fish like each other even in the same species. Just because you put them in a tank together does not remove the natural instinct to hate one another. This must always be considered when placing fish together. Consult a knowledgeable store owner for more information before ruling out a fish or combination of fish.