That's frustrating. My saltwater is a 32g BioCube, so I'm very limited on what I can stock, and I spent months looking up "nano fish" before even filling it with water! I love tangs, but my tank is too small for them, and so I haven't done a lot of research on them.
Posted some questions in your other post - mostly the same others will also ask. I work at a LFS, so hear about ich all the time, and the damage it does. Heartbreaking.
My store had a customer special order a bunch of sea horses, and everything was going fine. Then her AC went out (we're in the desert where summer temps regularly get up to 115 degrees) and she lost everything.
I hope you get to the bottom of it.