I am going through a similar situation in my 360. I will recap; please note that I waited at least 2 weeks between adding fish.
I set up my tank with 300 lbs of SD, 25 lbs of live sand and 300 lbs of live rock. I let it cycle until all my parameters were 0, then waited two more weeks. I started adding fish:
1) 4 chromies - no problem
2) 4 more chromies and a long nose hawk - no problem
3) 2 lobsters, a blue and a spanish, I had in another tank - no problem
3) a pair of cleaner, pair of coral banded and a pair of gold banded shrimps - no problem
4) a scopas and naso tan - no problem
5) a pair of lamark angers - no problem
then I ordered a pair of Japanese Swallowtails from the Marine Scene in California and all h*ll broke loose. I acclimated the fish as I always do and they hid for two days. Did not worry much because they were active. One morning the male showed signs of ich. I tried to catch it but could not; the next day the female had it. I almost climbed in the tank but caught them and removed them to a QT tank; they both died.
Within 8 days I lost the lamarks; within three weeks I lost the naso. I made two large water changes within a week (about 100 gallons total), waited three weeks and added an atlantic blue tang. It died in three days. The remaining fish and crustaceans I are fine.
I have made another large water change, about 60 gallons, waited three more weeks and I just added another atlantic blue two days ago. So far so good.
I don't know whether it was caused by stress, or the shipment, or something in my water. I don't know if it is a bacteria that has taken root in my rocks and will always be there. If this atlantic blue dies I am seriously considering emptying it and starting over.
Sorry about the long post but i wanted to let you know you are not alone.
Eddi