I thought of that, but I made sure the filter was empty of air (I encounter this in my aquarium canister filters all the time) and let it run for an entire day. I can see the air pattern. It is running fine for 10 seconds, then a short blast of bubbles. And repeat. I can rock the filter back and forth to purge all the air, then it's fine for a few seconds, then it builds up air again. Definitely sucking air, probably through the main gasket.

As for an internal filter, my goal is not to have to reach in the tank to clean it. I want to keep filter maintenance external. Means I'll have to step up to a better model.