When I was over last, you had LOADS of pods in your sump and if its a shrimp, it will "click" to stun/kill the pods and then eat them (a pistol shrimp will snap their claw at such a speed that it creates a very fast, hot and small bubble that will stun other living things). They also snap if they feel threatened, so if your lucky enough to get in their way, it feels like someone snapping a rubberband against your skin).
It could also be a crab or snail hitting their shell against the glass.
It could also be a crab or snail hitting their shell against the glass.
