155 gal tank issues with return lines

weird thing, on my 155 gal tank i have a sum below and i have the return lines that bring the water from the sump back up to the tank, Where the jets come into the tank i have noticed like every 30seconds or so it will shoot a bunch of bubbles into the tank, I thought it was doing that because the sump was low because its done that before when it was low so i looked and Nope, the sump is a little more then 1/2 full. Its not like its a constant flow. It will flow normal for a few seconds then it will shoot bubbles or whatever it is into the tank? Any ideas?

 

i have a spunge infront of the valve so it doesnt create a whirlpool, whatever it is whenever the fish swim through the return flow the stuff sticks to the fish. i just cleaned out the sump and put a new filter in it and when i pulled the filter out there was atleast 100 1/3 inch green bugs, I know theres bugs in tanks but iv never seen these green bugs before. They were sitting on top of the filter. I dont know if these little spots have anything to do with the bugs or not. Since i cleaned it out and changed the filter i dont see the bubbles or whatever it was coming back through the return lines.