The 1st year or 2 with a salt water tank will be your hardest. You will have to test a lot and learn water chemistry. My tanks are around 7 years old and I no longer test and I know how much chemicals to add every day.
Some of the best freshwater fish I had success with: Silver Dollars, Neons and Cardinal Neons, Buenos Aires Tetra. And I liked having one Betafish in the tank. As long as I had one, it was compatible with the other fish. It was a 45 gallon setup, was pretty easy to maintain. Biggest thing is changing out 10 to 20% of tank water on a regular basis, usually by vacuuming the tank bottom. With a 20 gallon tank, that's pretty easy.