My Blue Legged Hermit crab population has really lost decreased in the past 6 months. I don't know if some of them where a little bit older when I bought them or if they are fighting or maybe my Coral Banded Shrimps is killing them, but I need to get some new crabs because my sand bed is getting...

If I were to put a mantis shrimp in my quarintine tank would that be smart considering that the tank gets medicated at times. And I would also worry about smaller fish being a meal. Any thoughts or suggestions?