If the live rock was cured and you used live sand you may not get a big cycle. What kind of test kits are you using? What are you feeding the tank? An easy way to check for a mantis shrimp is put a piece of meaty food in a small container. A babyfood jar or similar works great. Turn the lights off and watch. It shouldn't take long for the mantis to "sniff it out". Do not kill the mantis. Some are quite beautiful. I'd set up a nano just for him. Or sell him to your local LFS.

Angie