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- Mostly BIG fish and some coral. ~80g Nuvo- My coral tank with "happy fish"~ 90g- FOWLR Not the not happy type of fish~ 125g- Freshwater Malawi Cichlids ~10g- Nuvo- The refugees from the Ich of '18