NG, you are really limited with your choices and how you did your drain. Bottom line is that too much air is mixing in from the drain, and the only real option is to deal with the bubbles in the sump. Most all overflows try to deal with air either by going solid... Herbie/Bean, or some sort of purging setup.. durso/stand pipe. You definitely DO NOT want to throttle your drain. That is not how you fix that. That is how you overflow your tank. In the sump you can do a sock like you have. If that is not good enough, you can use a sort of exit device. If that is not good enough, you can make some sort of chamber, baffle set up. Micro bubbles suck,and I can't stand them in the display. You can get it worked out. Next easy thing to try is this.... DO not glue anything, just fit it together. You put a 90 on your drain in the sump, then send that to a "T" fitting. One end of the tee is above the water. The other end is below. It helps to have the one below extended with some more pipe. the deeper, the better the bubbles stay on top, and not swept out the bottom. That's easy, because you can undo it all if it does not work. No need to glue, it will all stay together. Even bigger is better. Use larger pipe to slow flow. Flow velocity is what sweeps micro bubbles along. Any way you can reduce velocity, it helps. Also, only send as much to the sump as needed. what ever the skimmer needs, and the fuge if you have one. Anything over that is a waste.