Salt water tank in danger.?

Ok, so I really like tropical fish. My girlfriend bought her ex a fishtank for his bday when they was together and he recently decided to get rid of it and him being a nice guy gave it to her. Its a 55 gallon and we moved the water with it so it is already cycled and all that stuff. now he is the issue. ME being a idiot I am as soon as she said we was getting the tank I went online and order about 200 dollars worth of the first I wanted most. Very aggressive tank, lionfish,puffer,eel and a few others. Now we got the tank set up and ready for my new pets and when I got out the test strips the water tested unsafe nitrate levels. There is a clownish and a coral beauty in there right now and they seem to be doing just fine but I'm worried my new fish are going to die. The coral beauty has only been in there a few weeks and didn't die from shock of the new tank. Should I worry about my new fish and if so what can I do to help it.