1. You should always research the different types of fish you are going to be housing together. Certain fish can be extremely aggressive and cannot be paired with other fish of different temperaments.
2. You should never add uncured live rock to the aquarium. Doing so can release harmful levels of ammonia into your aquarium.
3. After establishing your aquarium, pick fish that will add well to the current water conditions in your aquarium. Also be sure to check their temperament and other needs to be sure that they will make the transition in your aquarium smoothly.