I agree with Todd, that is alot of fish for that tank. My guess as to the culprit would be the trigger. They are known to be nippers, friend of mine had one and all her fish had their tails and fins nipped at constantly.
I also have a 90g but only have a single tang, a small one too, a bristletooth which is really the only tang suitable for a 4 foot tank. You may have more issues in the future, just as an fyi.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.