It's funny you saw that. I added it literally 5 minutes before I took and posted the picture. I had used a "Herbie" style overflow on my 150, so I wasn't anticipating all the air going through the DT overflow into the frag tank... I was pretty bent when I first ran water through it and saw/heard the bubbles. All I did was take (I'll have to check on the size) the PVC pipe and cut 8 slits, 2 inches long, 3 blades thick, with my chop saw. I was getting mad bubbles, which were foaming on the top of the water. I still get micro bubbles, which you can see in the picture; but I really don't care for the frag tank. They don't carry on through the sump into the DT. You may try tweaking it with shorter slits or holes. I'm also going to try pulling out the 1in x 3pm pipe link that brings it under the waters surface in the bubble trap.. Your talking about your sump? Do you use a filter sock?
For my application, I'm concerned it's going to be a food or other surface debris trap, so imaging to have to rethink it and do a little research.