I just saw your previous post, where you have it in a 20H. If you just started the tank, and added the fish before it cycled completely, you have new tank syndrome where the tank is cycling with the trigger. Deteriorating water quality can bring on disease such as velvet, ich or bacterial infection. As people mentioned in previous posts, a 20 gallon tank is completely too small for your trigger, even though it is small.
Also, either a mantis would get eaten by the trigger, or it would kill the trigger.
I have a recommendation for you - before you get any more fish, read this book:
http://www.amazon.com/New-Marine-Aquarium-Step-Step/dp/1890087521
It will help you set up your tank properly and choose the right inhabitants for the size tank and filtration you have.
With a 20 gallon tank, you are very limited in the fish you can keep. You will be better off making it an invert tank, with either a mantis shrimp or maybe a shrimp goby/pistol shrimp pair.
I hope you can save the trigger. Your best bet is to find a very experienced local hobbyist to help you with proper treatment and water quality. If it survives, you will need to rehome the fish or upgrade to a larger tank. Good luck.