The Importance of Carbonate Hardness (KH) in your Koi Pond

The most overlooked, yet most important water parameter of your Koi Pond is KH. KH is the measure of the Carbonate Hardness in your pond.  Carbonate Hardness is simply the amount of Calcium Carbonate in our ponds. Calcium Carbonate has the chemical formula of CaCO₃

Ammonia, nitrite and nitrates as well as pH are important and they are the parameters usually measured in koi ponds. But why is Calcium Carbonate (KH), CaCO₃ so important?

Carbonate Hardness affects

The alkalinity of the pond. This has a stabilising effect on the pH.

The bacterial species responsible for ammonia conversion are referred to as the Nitrifying Bacteria. These bacterial species found in nature are chemolithotrophic autotrophs. They oxidize ammonia and nitrite to nitrate. They require oxygen and alkalinity for the process. They get their energy by the oxidation of the nitrogen compounds – ammonia – in the presence of oxygen.

Alkalinity is a measure of the buffering capacity of a pond. A stable pH is good for the koi and the filter.

For every gram of ammonia-nitrogen reduced to nitrate-nitrogen – 7.05 grams of alkalinity is consumed. The bacteria in your filter are literally chemically changing the ammonia to nitrite to nitrate but this action needs oxygen AND alkalinity.

Nitrifying bacteria grow everywhere and are affected by the alkalinity of the pond water simply because they function efficiently between pH 7.8 – 8.2. Nitrification stops to function at low pH – anything below pH 7 and the functioning of the filter bacteria becomes less and less to the point where they cease working.

Little or No Calcium Carbonate means NO BIOLOGICAL FILTRATION.  We know what will happen to the water without Biological Filtration and lots of koi - your water quality will quickly become polluted and toxic for your fish resulting in disease and death.

How does this work?

The ammonia in the pond comes from the nitrogen in the food we feed koi (after digestion they excrete ammonia) as well as the decomposition of organic material. Every pond has organic material in the form of algae (dead or alive) and biofloc – dead bacteria.
The koi breath in oxygen and breath out carbon dioxide – this is acidic.

We add oxygen all the time to compensate for the oxygen that the koi consume and the filter uses. But have we considered that Alkalinity in a pond is naturally decreased, over time.

Nitrification is an acid-forming process. The bacteria doing the work for us produce acidic compounds.

This will gradually lower the pH of your pond.

Rain is becoming more acidic. A pH of 4.5 was recorded in Krugersdorp, Gauteng, South Africa from a rain water gauge. Heavy rains will mean a drop in pH of your pond. Borehole water often is not well buffered.

There is a very good chance that unidentified problems in your pond could be linked to a very low alkalinity levels.

In study done in aquaculture systems, found that when the alkalinity fell below 100 mg/L (as CaCO₃) the ammonia showed a significant increase (to toxic levels). When the alkalinity was increased to above 150 mg/L nitrification resumed and quickly dropped to very low levels. (Recirculating Aquaculture – Prof. Timmons and Prof Ebeling).

“How Alkalinity Affects Nitrification”, Barillo, 2015

“During nitrification, 7.14 mg of alkalinity as CaCO₃ is destroyed for every milligram of ammonium ions oxidized. Lack of carbonate alkalinity will stop nitrification. In addition, nitrification is pH-sensitive and rates of nitrification will decline significantly at pH values below 6.8. Therefore, it is important to maintain an adequate alkalinity in the aeration tank to provide pH stability and also to provide inorganic carbon for nitrifiers.”

What if my Kh levels are too low?

The effect of low KH levels may not be immediately seen.  Low KH levels will ultimately lead to other water quality issues due to insufficient Biological Filtration.  A build-up of harmful Ammonia and Nitrite will start to accumulate in your pond.  Your pond will also be susceptible to harmful pH swings which could lead to a pH Crash which could kill all of your fish very quickly.

Prevention

Test the KH regularly and adjust the levels as needed.

Keep KH levels at about 150 – 200 ppm.

How?

Baking Soda - NaHCO₃ - can be used. Baking soda also known as sodium bicarbonate or bicarbonate of soda. Can be sourced from the local supermarket

How Much Baking Soda Should I add?

130 g per 5000 L daily until the KH reaches 150 ppm. Test 24 hours after the 130 g us added – before adding the next amount.

When Should I add Baking Soda?

As needed, whenever your KH levels drops below 100 ppm. Test after heavy rains.
Testing KH
Test kits for KH are available - look at the instructions - the test kit will read in
dGH (degrees of general hardness) - the reading should be between 6 - 8.
OR ppm (parts per million) - the reading should be between 100 - 150 ppm.
To convert dGH to ppm - multiply by 17.9.