1. 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.
2. It is not a good idea to wash your filter media with regular tap water. The tap water can kill the beneficial bacteria that is built up in the filter. Use distilled water to clean out your filter and its components.
3. 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.