Mu Yu is a form of Chinese percussion musical instrument, it comes in different sizes from DIA 40- 850mm producing different sound at each size. The smallest size is only for the used in the Buddhism recitation. The name Mu Yu (wooden fish) is believed to be a reminder for the monks, as the fish is awake and alert in the day and at night, Buddhism should be practiced just the same.
Here is what it sounds like with chanting:
And here is a Maywa Denki version: