Loosing fish dont know why

[Myka]

I'm thinking it's a mantis or pistol shrimp. Do you hear any "clicking" at night particularly when the lights first go out? I had a mantis in a 33g tank for 8 months before I found it. There was a 3" crab in a friend's 180g tank for over a year before he found it. So, sometimes things hide VERY well for a LONG time!!