Yes fish disease prevention and treatment is extremely important although a separate but related topic from cycling. With a 120G tank, that one seeded bio brick or whatever you got from your LFS isn't going to be enough of a biofilter. Add a bottle of biospira or whatever and you'll be good to go. If you don't have a separate QT tank, I'd recommend starting with a few fish in your DT and no clean up crew or coral. That way you can treat your DT if you see fish disease. However, if you buy pre-quarantined fish and start with a totally clean tank, then you don't have to worry about that. However, since you added something from your LFS already, you may already have ich or some other disease/parasite in the tank----so go for the fish only scenario first in my opinion