You don't need zeolite in a properly cycled tank. You will find it listed as a filter medium, and some filter manufacturers use it, but this is to remove ammonia from the water, the job of your filter. If you use zeolite, it must be placed at the end of the water flow so it doesn't remove ammonia before the filter bacteria have chance to get it or you will end up with a very small colony. The danger is here that if you forget to change the zeolite, once it is full, there won't be enough bacteria to remove the ammonia so the level in the water will go up.