For some reason I was thinking about gleet biofilters lately, and I've been seeing a few possible problems with them.
Could they sense the Yeerk in someone's head? If so, then how did they get uninfested people down into the Yeerk pool, and if not, then why didn't the Animorphs ever just try to go through a biofilter as humans? I think I remember something about how they only allowed through humans and Yeerks, not necessarily looking for a combination thereof, but the fact that Cassie had to morph Yeerk in #29 seems to suggest that they only allow controllers through.
And if they only allow controllers through, then how do they get new hosts in, such as when they capture people to drag them down to the Yeerk pool? If the system normally only allows controllers through, this would seem to suggest that the system can be deactivated to allow exceptions. And if it can be deactivated, then you can be sure that Ax, or for that matter any Andalite, can deactivate it. Not that they'd even have to. They could just morph human, two of them holding onto someone who's pretending to struggle, and yell, "Hey! New host, coming through!"
Next question. How did Erek get through the biofilter? Remember, that he did pose as a controller, and he technically even was one, since he had a Yeerk in his head. So he probably had to go to the Yeerk pool once in a while just to keep up the illusion. Wouldn't the biofilter have "seen" only a Yeerk, floating through the air with no host? And if Erek has a way to hide the presence of the Yeerk, that still means that whenever he goes through the biofilter, he doesn't register as human, or for that matter any biological entity. Wouldn't the Yeerks think that was a wee bit odd?
The other possibility is that he could trick the biofilter somehow. But if that were the case, then why didn't he ever help the Animorphs infiltrate the Yeerk pool? Being able to trick the biofilter would have been hugely helpful on a lot of their missions. And before anyone mentions the Chee's programmed non-violence, there have been times (#29, #37), where they've gone to the Yeerk pool on rescue missions, with no intention to take lives while they were down there.