I meant the reason for air tube into the sump but i think it is because you have k1 and using it to spin the k1.
For the air tubes, I should buy a new manifold/splitter for the airlines. The one I bought has 1/4" outlets for the airlines but all the airstones I use require bigger tubing, so I have to use 2 of the 1/4" outlets to connect with a T to 3/8" or 1/2" airline tube to go to airstones for the tank or the sump. Not a great setup but it gets enough air to the locations needed for now.
For the return pumps, I have 2 of them. 1 for the sump and 1 for the Ultima. Both use a reeflo dart/snapper hybrid. I have the sump set up with the dart configuration for slower flow and the Ultima set up with the snapper for slightly higher flow. I might change them over time but they seem to push plenty of volume for now. I have a bare spot in the front-right corner of my tank where the water pushes out of the return pumps and hits the front glass, then turns down to the bottom of the tank and pushes all the sand away from that location.