Clownfish are only aggressive against other clownfish (outside their group) and those fish they perceive as a threat to their anemone or eggs. Other fish are usually ignored unless they get to close or start provoking them.
Where the limits are differs from species to species and varies also a bit with the individuals. Interestingly enough i found clarkii (the ones I had) to be no more aggressive than your average ocellaris.
The nastiest clownfish I ever had were polymnus followed by sebae.
Premnas l actually found to be quite ductile and fearful. It may have been the setting I had them, but other clownfish i had in similar settings were not as shy and fearful.
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