I had a problem where the water entered the sump tank. It would pick up air on the way down which would come out below the surface of the water and bubble upward making noise. What I did to fix this was change the angle the pipe entered the sump tank. Instead of having the pipe go straight down, I angled it at 45°. This reduced the spashing noise, but it was still bubbling which was disturbing the water surface and wasting CO2. To eliminate the bubbling I inserted a 45° "Y" backwards into the angled pipe entering the sump so that the water kept going down at a 45° angle but the other end of the "Y" was on top heading back toward the surface. This allowed the air to escape but could only ripple the water inside the pipe. To further quite the bubbling in the pipe I capped the air escape pipe and drilled a few tiny holes in it above the maximum water level.

The hoses that connect the pipes in my upper tank to the pipes in my sump are just clear smooth Vinyl tubing bought at Home Depot.

Here is a picture of what I was describing. The left side is my new and improved plumbing and the right side shows my origional plumbing you can really see the difference it made.

Sump Tank Plumbing Picture