I guess this is stating the obvious but... you a have an air leak on the suction side. It's either in the intake, tubing, valve assembly on top, or the filter itself.
Unfortunately if you are using their tubes, they are opaque and you can't see inside to get clues.
One thing you can try is submerge the intake entirely (i.e. stick the 180 adapter and everything up to and including the first couple inches of tube) under water, where it can't suck air. That eliminates that connection. The one on the tank is harder, and the tank itself also. Check the seal there, maybe even remove it and re-lube (vaseline is fine in limited amounts).
Make sure the trays are well down and nested, and the cover that fits over them clicks down on it. If something is mis-aligned, the cover presses too hard up against the cover, and can warp the impeller cover (it's going to warp anyway - bad design -- but it warps sooner if you get it pressed up too hard).
If all else fails, it is painful to do, but plug up the hoses and somehow add 5-10psi pressure - maybe a big balloon you can squeeze, a tire pump on the end or whatever you need. Blowing in it by mouth won't be enough. Do it with it full of water, and see if any comes out -- an air leak becomes a water leak under pressure. You might also be able to use your other filter for this, somehow hooking the output of one to the input of the other (not running) and cover the far end (be prepared when something slips and water sprays everywhere).