Try a test add some kind of food like a small piece of shrimp wait till you get a nitrate reading (24 hours), then pull it out and see if the level drops all by its self.
This kind of test can really go bad if you over feed the tank try for 5-20mg/L nitrate and keep some Prime handy if you over shoot. Dose the Prime to remove most not all the nitrate that way you can see that the bio-filter is working; when/if the level drops all by its self. Now if the nitrate keeps rising I would dose Prime to keep the level below 20mg/L others may say you don’t need to do that.
The objective is to prove that you have a large bio-filter if you don't and you add a fish the ammonia and nitrate will spike, and then after that drops you will have a large bio-filter but possibly a dead or at least a stressed fish. You can also get an ammonia and nitrate spike if you stock a tank to rapidly.