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. 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. It is generally recommended to use natural gravel for the aquarium. Colored gravel can be used, as long as it is professionally made, certain cheaply made color gravels can be toxic to fish in your aquarium.