The 500 is also suitable for Sea fishes and coral reefs. Sea fishes and coral reefs needs more attention, it's not as easy as fresh water tank when it comes to maintaining it's environment and health of the sea water. My advice is to start with as little Sea fishes and Coral reef as possible, get books on this subject and learn thru the stages with patience and calmness. Tropical sea fishes and coral reefs are expensive, it hurts when all is not working out well for you, just a bit of advice here.