A Quick Understanding of Chlorine and Pond Filters

Read so far

A Quick Understanding of Chlorine and Pond Filters

February 20, 2018 - 12:48

Article by Paula Reynolds

Tap water contains chlorine, a disinfectant that makes water safe for humans, but is harmful to fish. It is removed with a purifier, a dechlorinator, or a water treatment product from an aquatic outlet.

Chloramine is chlorine combined with ammonia and this is introduced to the water supply in order to ensure that the chlorine travels through the pipe work that delivers water around the country. Chloramine is harmful to Koi and can make them ill very quickly. It is not removed from water by all the devices or products that are sold to remove only chlorine. In order to neutralise chloramine a tap water conditioning product is required, such products also remove metals and other harmful constituents of water. When introducing tap water to a pond; use the amount of water treatment product for the water added and not the entire pond gallonage. Positioning a hosepipe above the pond allowing tap water to cascade into the pond to dissipate chlorine or chloramine is a myth, this is not a satisfactory method of making the water safe for Koi. In a new pond filled for the first time before fish are introduced, the water has to be turned over for two weeks in bright sunlight to make it safe, although, only testing the water will reveal how safe it then is. Test kits will reveal dangerous levels of either chlorine or chloramine but they will not trace lower levels, although, many hobbyists use DPD4 tablets that will reveal both in the same test. There is no level of either chlorine or chloramine that is safe, so, preventing exposure is the best approach.


It is vital to keep Koi in a pond that has a filter system adequate for the number of Koi stocked and the pond gallonage. Koi create more waste matter than other pond fish, and without filtration, they would not survive. It is only in large water volumes or with very low stocking levels that it is possible to keep Koi in unfiltered water and hygiene can ultimately become an issue in such ponds. There are numerous bacteria involved in pond filtration and certain strains cannot survive in every type of filter. The accessibility of light, the water temperature, the speed of the water flow and the type of filter media and other factors can be a disadvantage when it comes to forming a biological balance that fully supports the water chemistry. The primary problem with filtration is that the decomposition of fish waste creates acidity and much depends on the bacterial strains, their volume, and their capacity to survive in acidity. If the pH drops to 6 it is possibly crashing and the fish will react knowing they are in danger. However, at a pH value of 7.5 to 8 there should be enough alkalinity to buffer the acid that is being produced, as it is only if this balance is lost that the filter will crash.

Source: Paula Reynolds, Bsc, PhD,BA - Paula an aquatic patho-biologist, runs Lincolnshire Fish Health Laboratories and Research Centre a company that specialises in research into fish diseases for vets, fish farmers, hobbyists and the aquatic trade in England.


!If you're not havin' FUN, you're not doin' it right