50% water change and still 20ppm nitrates

[o.c.d.]

I ask because when you remove po4 quickly out of the tank the bacteria feeding on it will die then cause a spike in nitrate. Even large waterchanges to me are not a good idea. slower is better.
Now zeobak isn't that the food source for the bacteria that live on the zeolite? If so by stopping dosing you quickly removed the food source maybe causing the same effect.