Yes I am using a prefilter because I have baby endlers. I can see how this might be the cause. There are large air bubbles that build up on the prefilter(filter floss) that probably get sucked through and dont diffuse. This is possibilty.
I have 1x xP1 and 2. I haven't had any problems with the air and prefilter with those. I normally use these
Filter Max prefilters. From what I can gather, it seems that the extra power/draw of the xP3 is bogged down by the prefilter and--I assume--causes air to "leak" in past the seal on the unit. There may be some other explanation, but that's the conclusion that I've reach over the last yr+.
That said, the prefilter on the xP3 on my 55g will clog enough to collapse in on itself and basically stop the flow--not 100%, but heavy duty!
I haven't had this problem on the 2 smaller ones, but was a thought to pass along. I just setup a 2nd xP3 and had the same air problems. I swapped out the prefilter for the xP strainer--and the problem went away....
I really like the Filter Max prefilters, but it seems that they are not really compatible with the xP3. They are interchangeable with the Hydor sponges, so I may try a larger size sponge.
Finally, I have them and I have not tried it, but I would assume that the coarse sponges for the Marineland reverse-flow powerhead setup would work better. They do a Good job. Clog fairly quickly and all--but I haven't tried them on the xPs.

roud:
HTH