I too have heard a knocking sound at nights. After reading these posts I was was convinced i had a mantis shrimp in there too!
Anyhow i have had the day from hell as a result. I decided to strip the tank down and dip EVERYTHING. Everything got cleaned, dipped and syphoned.
To my horror, i found 3 shrimp things that flushed out, along with a bristle worm and a small crab.
It has taken me over 6 hours! Anyhow thankfully everything is back in the tank now, re-aquaacped and the lights are off, i can't hear any more damned clicking sounds so hopefully whatever it was is flushed away and the demon hitchhikers are returned to Dante's 5th circle of hell and not in my tank! LOL
My mantra now is .....I must stop reading mantis shrimp / bristle worm / hitchhiker posts.
:smack: