Hi and welcome aboard Robert,
A 20 gallons would certainly limit your choices. Live rocks will do the cycling for you.
Salinity for marine is usually 1.025. Hydrometers will measure that.
I'm guessing the bubbly thing is an airpump which is not necessary if your filter makes turbulent movements.
Other equipments to consider are sump(not necessary though), thermometer(to check temp), protein skimmer, etc.
This thread will surely help you for shopping list.:thumbsup:
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