You're "good" until a parasite makes it through the tap, gets on you and into the tank, is introduced by another fish you just added to replace the dead one(fish don't live forever) and parasites take many forms. Some fish will pass parasites to other fish without even showing signs of an infestation. Some fish, and its been argued that ALL fish, have parasites in their gut passed down for many, many generations. The best defense against a parasite is good fish keeping and a good contingency plan.
Always keep RidIch and Prazipro(or similar) on hand. The best and most balanced tank just needs the right set of circumstances to make it all go wrong...sometimes over night. Plan for the worst and hope for the best.