With much of the experimenting over on the Ornamental set, it time for reducing its outlet sound. At times the siphon outlet can be loud and it's no different on this smaller set. The easiest way to reduce the sound is to put a muffler end on the outlet.
Muffler end is made by drilling few holes on the outlet final downward tube , this works as to let air water mix and thus reducing the sound.
This muffler also act as an aerator giving much needed oxygen to the water. If needed to reduce noise and giving oxygen to water, this is the simplest way to do it.