Many Koi keepers will tell you that you need to add clay to Koi ponds. Bentonite or Mortmorillinite clay are what's sold as Koi Clay. Here's a short article by Syd Mitchell about benefits and disadvantages of using clay. If you can't come to the PNKCA Convetion June 19-21, 2015 - then take our ON-LINE Lab #220 from the comfort of your own home! CLICK HERE for more info about the lab. Full article after the break.
There isn’t a definite answer to whether clay is a necessary additive to Koi ponds, it entirely depends on the exact make up of the water chemistry but it does have benefits.
Koi, and other fish, need a wide range of minerals and trace elements in their diet. Food manufacturers try to ensure that their product contains everything a Koi needs but fish also take in dissolved minerals direct from the water by absorbing them chiefly through the gills and gut.
Carp will eat whatever they can find. In the wild, they find a wide range of food which helps ensure that they obtain everything they need to stay healthy. Some of this food is found by rummaging through the mud at the bottom of their lakes which means that, when worms or insects are eaten, some of this mineral rich mud is also swallowed. Rotting organic material from plants or even dead fish is also broken down by bacteria in a process called mineralisation which releases minerals back into the water to dissolve and be available to be absorbed by other fish.
In contrast to a natural pond, we try to keep Koi ponds as clean as possible. We don‘t allow detritus to build up and then decompose into its component minerals so it becomes our job to supply our Koi with everything that they need.
When a pond is filled, it contains minerals from the mains water supply unless they have been removed by a purifier. Water changes and top ups help to replace those minerals as they are used but there is a difference between water in which Koi can survive and water in which they can thrive. This leaves the question; will water changes alone add sufficient minerals and trace elements, and will these be the correct ones anyway? Pond clay can ensure that pond water doesn’t become mineral deficient.
Minerals introduced by adding clay vary according to where it was mined. Bentonite clay is particularly rich in calcium. Calcium cannot be synthesised by fish and is vital for strong bones and scale development, it is a catalyst for enzyme action and other metabolic functions but it has other benefits as far as pond water is concerned.
Calcium has what chemists call “a negative electrical charge” which means that as this type of clay settles slowly to the bottom of the pond it will attract positively charged impurities to it. Although calcium molecules don’t actually remove impurities from the pond water, nor can calcium make the impurities vanish, they will be permanently locked together and, as such, they cannot affect the fish. Then, when the clay is filtered out as sediment in the filter bays and flushed away, it takes those pollutants with it. In this way, toxins such as heavy metals, free radicals, and pesticides are removed from the pond.
There are other benefits to pond clay but there could be a drawback if it is used excessively since all pond clays increase both pH and hardness. Koi are tolerant of a wide range of pH and hardness provided the actual levels are stable and do not vary abruptly. They are particularly sensitive to variations in pH of more than 0.2 per day, so if clay is used, the recommended dose rate shouldn’t be exceeded.