Even modest levels of ammonia are a threat to fish health. This is particularly a problem in new ponds and aquaria where we have to let ammonia levels rise a little to encourage the establishment of nitrifying bacteria in the filter.
Elevated ammonia can occasionally be a problem in established set-ups.
These are naturally occurring types of minerals, although because of their usefulness as molecular sieves they can now be artificially produced. In their natural form they are aluminosilicates, which are normal silicate minerals containing aluminium. The structure is such that these minerals can loosely bind positive charged ions, usually sodium. They act as ion-exchangers and 'swap' their sodium ions, for other positively charged ions, for example calcium or ammonium. So they are often used for water softening to remove hardness-forming calcium ions.