wright when it pours back in the main tank there should be no bubbles unless like you said.............your water level is to low. that can get messy. you will have salt crust all over the place. i would buy a extention to bring it down in the water just to keep that from happening. plus it cause the more and more noise. you hear that splasing water hitting your surface. if this is not that case i would say you have a little air getting in somewhere. what is you return pump? is it submerisble or a external pump?