Well, just because the ammonia goes through the media where the bacteria grow first, doesn't mean it will all be captured and used there. Even with a fully grown bacterial colony, a lot of the ammonia would still go through it uncaptured. The issue comes in the next step where the zeolite is there to be a perfect capturer -- that's why it works so well at what it does. Zeolites are amazingly good at capturing 100% of what they are going to capture until it gets saturated. There is a little wiggle room when the breakthrough occurs -- it depends on the velocity of the fluid carrying the adsorbed molecule, but the idea remains the same -- that all of the ammonia that goes near the zeolite is going to be captured.
That doesn't allow any to come back through the filter. Basically, in your scenario, the bacteria have one chance and only one chance to capture the ammonia for nutrition. After that, too bad. The bacteria won't grow to any significant size like that.
Since you asked/commented that water changes act just like zeolite, I'm assuming you have not read my article, so I'll link to it for a third time here.
http
/www.fishforums.net/content/forum/22...cling-Are-Good/
However, zeolite and water changes are not the same.
Zeolite would add an extra sink term into the balance equations. The accumulation equation would now look like this:
a = r_p - r_c - r_z where r_z if the rate of adsorbtion by the zeolite.
r_z would be proportional to the current concentration of ammonia if the zeolite isn't saturated, and equal to zero if it was.
Because r_z would be such a large sink term at the beginning, and r_c (the rate of consumption of ammonia by the bacteria) is so small at the beginning, the zeolite effectively blocks out r_c completely. The equation, at the beginning becaomes, a = r_p - r_z. And, if r_z is at least as large as r_p, there will be no accumulation, a. So, there will be no incentive for the bacteria to grow.
Again, this comes from the fact that the ammonia goes through the bacterial colony over and over and over -- this is a primary reason you want the filter to overturn the tank several times per hour. You bring the colony food constantly. With zeolite, the bacteria have to eat it on the first pass or they get nothing. They aren't going to take up very much food at all like that -- bring the nutrients back over and over again lets them grow continuously. To put in terms on an analogy, it would be like sitting you down in front of a giant buffet and telling you that you can eat all you want in this sitting -- but it's the only meal you're going to get for the rest of your life.
Finally, regarding the leaching out, it is tough to say because I haven't studied it. Nature is always striving for an equilibrium. In the first place, the concentration of ammonia in the water is high, and the concentration of ammonia on the zeolite is low. Since the zeolite attracts the ammonia, the ammonia will leave the water and attach itself to the zeolite. Most zeolites are very attractive to the molecules they adsorb. However, once the water is free of ammonia, the concentration of ammonia in the zeolite is high and in the water is low. So, again in always striving toward equilibrium, a few molecules of ammonia will break free of the zeolite and reenter the water. It was a bit of an exaggeration above to say that zeolite captures all of the ammonia in the water -- there is an equilibrium concentration. That is, given an initial concentration of ammonia, the zeolite will adsorb a certain percentage of the ammonia from the water. The reason I said "all" is because zeolite is usually very good, well over 99.99%. Essentially it is all. But, in the same way, if there was saturated zeolite and clean water, the zeolite will release a small amount to get back to that equilibrium split. Again, its only going to be a tiny amount, probably concentrations in the part per billion or even parts per trillion range.
Zeolite adsorption is reversible, but the issue is the activation energy of the reversal process. Usually, it takes really hot temperatures and higher pressures to swing the equilibrium towards having the zeolite release rather than attract. The amount of it's desorbtion in the fishtank has been overstated. The amount of desporbtion of carbon, however, I think has not been overstated.
So, to summarize: the issue at hand is that the zeolite does too good of a job attracting and holding on to ammonia. Even if the media where the bacteria are to grow is in front of the zeolite, giving the bacteria only one pass to consume the ammonia is insufficient for good, constant bacterial growth. They are not the same as water changes, because water changes dilute down the ammonia, but there is still some ammonia going through the filter, getting to the bacterial constantly. Zeolite does not give the bacteria that chance.
