Zeolites (Sodium aluminosilicate, zeolite A) are minerals that have a micro-porous structure. There are used to remove certain chemicals from water. In the aquarium these small white rocks are used to remove ammonia from the water.
Usually added to an aquarium in an emergency to quickly remove ammonia, a quantity of the rocks are placed in a media bag so it can be replaced afterwards, or simply scattered into the tank as the rock is harmless if left permanently (as long as you never add salt to the tank). It will provide a large surface area for nitrifying bacteria. 1 gram of zeolite will remove 1.5 mg of ammonia.
The product can be recharged by the use of ordinary table salt. Take a litre of 5% solution of Salt water and soak the zeolite in it for 24 hours. Then rinse afterwards.