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!!
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~ Mindy
SPS fanatic.
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