There is a mineral which selectively removes ammonium from the water. It is a zeolite called clinoptilolite. This can be found in the LFS as white granules. This material can remove about 1.74 mg NH4–N g−1 (“Ammonia Removal from Leachate solution using Natural Chinese Clinoptilolite”, Wang et. al. 2006).
A 100-gallon aquarium with 3 part per million ammonia in it, has roughly one gram of ammonia in it. One gram of ammonia will require 574 grams of zeolite, or over one pound of zeolite, to remove the ammonia. Considering that this costs about $20 and an 80% water change would do the same thing, as would a functioning cycled biofilter, I’m not too sure where there is much of a use for this material.
Source: Aquariumscience.org – David Bogert