Sorry to hear your having troubles, take things slow for now and try and figure out what the problem is and work on a cure. Add no other fish to the tank until things are under control.
I'm no expert on fish disease, but I would say you may have some type of bacteria problem.
what type of water are you using? tap, ro
.. I'll let some others who may have more experience with illness jump in. I would think about running a UV
light if you have one or can borrow one.
the tank set up sound good so you are doing well on that end. Salifert test kits are easy to use and IMO
are some of the best.