Dictionary of Ichthyology  

zeolite = an ammonia-removing, ion-exchange mineral used in freshwater aquaria. It also can act biologically and mechanically. Does not last long and must be regenerated by soaking in a strong salt solution for a day. Particularly useful for temporary ammonia problems, in hospital tanks where the chemicals are harmful to biological filters and in transporting fish on long journeys.

Wikimedia Commons: http://commons.wikimedia.org/wiki/File:ZeolitesUSGOV.jpg

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