Sorry about the loss. I went through a "bad streak" when I was first stocking my tank. Never could really figure it out, except for perhaps a small chunk of live rock I picked up from a LFS
that I had in the QT. Once I took it out, everything lived after that point. Go figure.
Sounds like you had some type of parasite on him, and it sounds like you didn't do anything wrong - just bad luck from the store. (Your pH is a little low, but I'm guessing you know that.) Either way, I'd empty the tank out, give everything associated with it a good bath and let it air dry for a couple days. Then fill it up and try again with a new sponge filter from your main. And maybe try a different store... even though your friend works at that one.