It's essential to replace cartridges in your purifier regularly because the materials become exhausted and ineffective after a certain period of time, depending on how polluted your water is to start with.

Ensure that you use the recommended flow rate through the purifier cartridges. If the flow's too fast the purifier will not be able to remove all the toxins. And, as ever with Koi keeping, you need to consistently monitor your water quality. Obtain regular water company reports, test your tap water and the treated water using reliable test kits or a specialist lab. Don't wait until your fish begin to suffer to think about what your water's made up of.