Clicking in my tank

[blood_hound]

Every once in while I keep hearing this clicking sounds in my tank, most of the time during the night when main lights are off.I been hearing that sounds for almost 2 months now. What could it be?
at first I thought it was a mantis so I start counting my shrimps and snails and their all still there alive and well. And i also observe the mantis that I have and it doesn't click that often not like the one that I hear from my main tank.
And I also thought maybe it is one of equipment, but no.


WHAT COULD IT BE?