Chemical filtration uses specific types of media whose surface area has the ability to chemically bond with pond pollutants, essentially absorbing them and removing them from the pond water. The pollutants which are absorbed by chemical media are; ammonia, nitrite, chlorine, organic compounds and some chemicals. There are two chemical media which are commonly used in pond chemical filtration.

Activated Carbon

Activated carbon is available in the form of charcoal granules or pellets, and foam pads. It is produced by heating carbon-rich materials (nut shells, bone, wood, peat, etc) to temperatures as high as 1200OC in the absence of oxygen.

The resulting carbon becomes porous and is filled with millions of micropores, giving it a massive surface area. As pond water flows over its surface the carbon absorbs the dissolved pollutants, making the pond water much healthier.

Activated carbon has a limited amount of contaminants which it can absorb, and it must be changed regularly. Most carbon products have a life-span of 2 to 3 months. After this period, the carbon’s surface becomes covered with pollutants and bacteria and cannot absorb any more toxins. Once saturated, the carbon can release all of the absorbed pollutants back into the pond water.

Zeolite

Zeolites are naturally occurring microporous minerals which are more commonly used as a chemical filter media in larger ponds. They are essentially rocks which, like activate carbon, can absorb toxins from the pond water.The main benefit that zeolite has over carbon is that it can be reactivated. Zeolite which has been absorbing pollutants in the filter for around 2 months should be removed from the filter and submerged in salt water for 24 hours. Any pollutants are drawn from the zeolite, which can be rinsed in fresh water, and returned to the filter.As with activated carbon, if zeolite becomes overloaded it can potentially release all the absorbed pollutants back to the pond, which can cause a number of problems. Also, zeolite prevents the pond owner from using salt as a pond treatment because the added salinity will cause the zeolite to release absorbed pollutants back into the pond water.

Chemical filters also have the ability to absorb beneficial chemicals from the pond water. This will hamper the effects of many fish and pond treatments.Because they have such an immense surface area, chemical filter materials soon become populated with bacteria. It is important to re-colonise the pond bacteria with additives when replacing activated carbon or reactivating zeolite, as the loss of so much bacteria can cause a sudden rise in the amount of ammonia in the water.