I bought an AquaOne 2400 earlier this year and Am now just recently having issues with air getting trapped inside it, Just by coincidence I was in the room when the motor started making a weird dud dud dud dud dud sound, checked it out and the water was no longer passing through it - Too much air had collected inside.
So i re primed it and away it went, later that day i came back to it and rocked it back and fourth to check for more air and sure enough a bit came out (and dirty water).
Anything I should be checking? If it was the seal surely there would be a visible water leak??
I cleaned it about 2 months ago so there shouldn't be anything blocking it up.