Using Chemical Filtration in Your Pond

Using Chemical Filtration in Your Pond
Besides mechanical filtration, which is a means of removing suspended particles and other debris from your water feature by literally straining it through a physical filter, there is another type of filtration that is popular with aquarium enthusiasts and pond keepers, alike. It’s called “chemical filtration.” The name is a bit misleading, since the “chemicals” involved are not added to the water, as you might think. Instead, you add something to the water in order to absorb unwanted chemicals, particularly ammonia, the most harmful to plants and fish.

The most common form of chemical filtration is carbon, sometimes referred to as charcoal. Carbon helps to clean the water through absorption. Millions of microscopic pores on the surface of the carbon absorb ammonia, a potentially toxic organic waste, and and hold it deep inside, where it can’t do any harm.

The drawback to a carbon-based system for use in a pond is that carbon becomes saturated with absorbed chemicals quickly, after which it actually begins releasing ammonia back into the water, which is not a good thing. (Ammonia can be deadly to both plants and fish!)

Rinsing the carbon in order to clean it doesn’t work, since the chemicals are locked solidly within the carbon for good. What’s more, it’s impossible to tell when the carbon is saturated and no longer doing its job. This negates any really positive values carbon might have for use in pond-keeping, although if you have unlimited funds (and don’t we all!), replacing the used carbon weekly is a good possible solution to the problem.

Another material, Zeolite, works similarly to carbon and can actually be cleaned, or “recharged,” by soaking the material overnight in a bucket of salt water. The salt draws the ammonia out of the Zeolite, thus enabling it to be used again. The problem with using Zeolite is that, as in the case of the carbon, it’s impossible to tell when the Zeolite has reached its ammonia-saturation level. Also, while it might be suitable for use in a small pond, the more water you have to treat, the more Zeolite you have to lug from the pond to the saltwater bath and back again, making it less than ideal for use in all but the smallest ponds.

The solution? For smaller ponds, you could have two equal amounts of Zeolite on hand, swapping the spent material with the recharged one weekly. For larger ponds--well, we'll talk more about still other filtration options available to you later.

Check out D. J. Herda's two latest gardening books, Zen & the Art of Pond Building and From Container to Kitchen: Growing Fruits and Vegetables in Pots, both available from

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