1. It is a good idea to keep carbon on hand in case of emergencies. Carbon can be used to soak up anything harmful that may be introduced into your aquarium. So keeping an emergency stock can be a life saver.
2. Only use medication in your aquarium when you are out of other options. You should use a separate tank to medicate sick fish as some medications can have harmful effects on healthy fish.
3. You should never add uncured live rock to the aquarium. Doing so can release harmful levels of ammonia into your aquarium.