@AllieBdot
In my view, it seems that there is more than one issue, the biggest being a lack of quality guidance with your first tank.
1. API Test Kits: In general, the consensus is that API test kits leave something to be desired where accuracy is concerned. Personally, I use them as a way to do a quick test to monitor water conditions. And I use Red Sea test kits to confirm the API results when API is reporting something of concern. API will report the presence of ammonia, even when none exists.
2. Quarantine: In my experience, there are no circumstances under which a new fish should go directly into a display tank. Personally, I monitor a new fish purchase in quarantine for 90 days. And if they show signs of disease, I reset the 90 day clock once treatment is completed.
3. Go slow: There is an expression that those of us who have been in the hobby are familiar with; "Nothing good happens fast in this hobby". True. You MUST be patient. There are many things in life which we can own impulsively, but a reef tank isn't one of them. Unlike a new 60" TV, you don't pull it from the box, plug it in and enjoy.
Think of your reef tank as a garden; something that needs to be nurtured into the rich colors and aromatic blends we enjoy. It doesn't happen overnight.
4. Last: Based on your picture only, I would say that your tank is capable of supporting fish. But if you plan on adding corals at some point, you may want to consult here before doing so.
Please post a detailed list of your tank equipment so that proper guidance can be provided moving forward.
Good luck!
Dom