Don't give up! The good news is, you can overcome the problems you are having. The bad news is, it will take a little work and making some hard decisions.
Based on your readings, you have some water quality problems that are causing a lot of your issues and stressing your fish. Your tank is not cycled...either because it never was, or because something has kicked it into a mini-cycle. Are you using
RO water, or tap? If tap, are you treating for
chlorine/
chloramine? Looks like you have no
live rock. What kind of filtration are you using? You need to have someplace for the beneficial bacteria to grow...either filter media or live rock. Live rock is the best, and I would highly recommend adding 1-1.5 lbs per gallon of water (be sure it is cured first!). I would suggest another
water change as well. I prefer once per week, even with a fully cycled tank. More frequent may be necessary in the short term until your tank cycles. Bottom line, you need to get those readings in line...the ammonia and nitrite can kill your fish outright, or at least stress them and increase their suceptibility to disease, as you are seeing with the tang (tangs are prone to ich anyway...stress makes it worse).
Something else to consider...Triggers are aggressive with tankmates, and as you are seeing, they can damage your other fish. This will be more of an issue, the smaller the tank they are in. You may want to consider getting rid of the trigger so the other fish can live in peace.
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