Some fish are more resistant to disease and parasites, though still able to get it.
Wrasses and clownfish produce a relatively thicker slime coat that is more difficult for some parasites to penetrate.
Fish that are hardy won't get stressed out as easily as more delicate fish, and when stressed fish's immune systems don't work as they should.
Other methods, such as ttm can be utilized to guard against particularly virulent pathogens.
I don't use copper with wrasses, though I am beginning to suspect their sensitivity to copper is in part to misdosed copper, or a new wrasse's frailty, rather than the copper itself.