you can also go to a pet store and buy one of the clear critter cages with the colored tops. They have a door hatch in the top and work great as a trap.
just go to youtube and search for a fish trap make one out of a small water bottle or 2ltr bottle depending on fish size. They work awsome. leave it in tank a few days without feeding the tank then put food in it. And hope he is the first one in
Ohhh you probably have a mantis shrimp in there. They make a loud clicking sound randomly. They WILL be predators in you tank and can kill even bigger fish. You'll have to catch it somehow, google mantis trap.. GL
up the feeding for the time being and definately do what you can to supplement what your feeding the tank. Also try to make water changes the least dramatic as possible, cause they can be stressiful if done quickly or massively.
ill shoot you a pic tonight of what i have. I'll let it go for like 2 a lb. maybe a group bundle or something if you take it all. I have no idea of the weight but its like 10-12 med pieces