1. 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.
2. It is not considered a good idea to keep aggressive fish with fish of other temperaments. You should keep peaceful fish in an aquarium with other peaceful fish to avoid any sort of fighting between the fish. Aggressive fish should only be kept with other aggressive fish.
3. You should keep the temperature in your aquarium constant. Constant change in the aquarium can be dangerous for the fish in your aquarium. Keep a quality heater and accurate thermometer to ensure that the water stays stable for the aquarium.