Check out the threads on this site to determine specifics. Get your filter name off the unit, the company name off the test kits.
Test the following in this manner.
Ammonia, nitrite, and nitrate
Daily for a week
Weekly for a month
And a few times a month after that.
You need to learn the nitrogen cycle, do a search, and how it will affect your clown fish. There is something called cycling that needs to happen and can seriously injure or kill your new fish.
Pick up some Prime to help with the cycling process or return the fish and start fresh in a few weeks after the cycle. Get more salt and prepare for water changes that will be necessary if you decide not to return the fish for the short term.
One thing about saltwater vs freshwater. Totally different and go slow. Go slow, good luck, and read read read. A fifteen gallon is small which means a higher chance for crashing.