HOW DO YOU KNOW IT IS WORKING ?

You can tell the POLY-FILTER is working by observing its color gradually changing from an off-white to a brown color after prolonged exposure to contaminants in your aquarium. Cut POLY-FILTER in half when dark brown in color. If cut cross-section is also dark brown replace with new one.
Another method is to check ammonia levels in your aquarium before installing the POLY-FILTER and periodically thereafter.
A third method is to add medication and subsequently check its concentration in the aquarium water after approximately 48 hours.