Mantis shrimp! Help

[windcoast reefs]

So I just put my first fish in my tank on monday (green chromis) and they seemed to be doing fine. They adjusted allot quicker than I expected for a new tank but yesterday I lost one of the fish. On tuesday morning I noticed that one of them had a huge gash on its side and a lack of fins. It was right by its tail on ins right hand side. It died very quickly after I noticed. But today I woke up to my other green chromis with exactly the same issue! The same gash the same spot and no fins. So I started to investigate and had quite the surprise. I say a inch and a half long mantis shrimp on the live rock! I had noticed this in the tank when I set it up in september. I had no idea what it was, but I did notice that is was super aggressive (put a whole right through my glove and killed all the other tank makes at the time). I had thought that I managed to kill it off by taking the live rock it lived in out and then covering it in carbonated water and doing a few water change over the next few weeks. I haven't seen it up until now. So does anyone have any advice to get rid of this mantis shrimp? I figure they may have been hiding at night right by the live rock that it lives in. I didn't think it could live out of water for 24 hours and be covered in carbonated water. So I am out of ideas.