1. 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.
2. It is never a good idea to add shells from the beach into your aquarium. Introducing shells raw from the beach can add harmful bacteria and potentially new creatures to the aquarium.
3. When starting out a new aquarium, it is best to add hardier species at first. Once your aquarium ages a bit and gets settled, you can start to add more sensitive fish to the aquarium.