Muffler for LED "fish lamp"

The LED has a water filled tube with some fish made of plastics, where a membrane pump generates a stream of bubbles, which in turn make the fish float up and down. Unfortunately the pump resonates with the tube and gives quite a noisy hum at multiples of 50 Hz up to about 400 Hz. 
The muffler suppresses this by about 20 dB, so it seems to be quite silent as long as there is some other noise. 

To get it air tight the parts need to be glued together. The blue adhesive tape around it was just used to fix it until the glue is dry.

I'm still not satisfied, because the thing still hums at around 40 dB. Probably I need to dive deeper into acoustics or modify some of the existing muffler for a better effect.