I hear that a chopstick shoved into the airline and cut off makes a great diffuser, since the bamboo is super porous. If you want something that is more like 100% diffusion, try putting the line close to the HOB
filter intake (if you have one). The impeller will chop up the bubbles nicely. Also, be sure that the water level is nice and high, so the water coming out of the filter doesn't break the surface tension.