Some "cleaner fish" consume parasites and mucus from the skin of larger fish. what would be a reasonable argument for why this could be a mutualism rather than a commensal relationship?

This could be a mutualism rather than a commensal relationship because when "cleaner fish" consume parasites and mucus from the skin of larger fish, they don't only receive food from the relationship, eliminating parasite and such from the large fish, but also they gain protection from predators, thus making the relationship a good example of mutualism, because both parts of the relationship gain something from the relationship itself. 

A- a reduction in biodiversity