Machine Aquarium What Else Would I Need For A Saltwater Aquarium?

What else would i need for a saltwater aquarium? - machine aquarium

I
a tank with 20 gallons
Green Killing Machine
Heaters
an air pump
Built-in lights
3-stage filter construction

What do I need for a saltwater aquarium and can products and low cost must be recommended.
I read a lot about salt water aquariums, so I know what I'm doing.