1. Having a quarantine tank on hand is a very good practice to use if you frequently buy new fish. The quarantine tank will weed out any sick fish that could possible get into your aquarium and cause harm to the other inhabitants.
2. Keep plenty of testing kits on hand, if you own a saltwater aquarium you should mainly focus on nitrates, PH, salinity, and phosphate levels. Never be caught off guard, make sure you have plenty of kits available to test the levels.
3. You should never add uncured live rock to the aquarium. Doing so can release harmful levels of ammonia into your aquarium.