Testing for Carbonate Hardness

Pond & Lake Q & A

Q: Why do I need to test for carbonate hardness in my lake or pond? – Jose in Michigan

A: A number of pond treatments, including some algae-control formulas and other herbicides, contain copper. In certain conditions, the copper chelate may break down and release copper ions, which could cause your pond or lake’s alkalinity to rise.

If you use these treatments in a pond or lake stocked with trout, koi or goldfish that are sensitive to high alkalinity levels, you need to test your hardness regularly to ensure the health of your fish population.

What is Carbonate Hardness?

Carbonate hardness, or alkalinity, is the measure of carbonate and bicarbonate concentrations in your pond or lake’s water. Alkalinity is a measure of the ability of a solution to neutralize acid without changing the pH. It both controls and maintains water pH.

Alkalinity is related to the amount of dissolved calcium, magnesium and other compounds in the water, so alkalinity tends to be higher in harder water. It naturally decreases over time through bacterial action that produces acidic compounds that combine with and reduce the alkalinity components.

In a pond or lake, the alkalinity of the water is critical to the health of the fish – especially for trout, koi and goldfish. For these fish, the carbonate hardness of your pond or lake must exceed 50 ppm for the fish to survive, ideally falling between 50 ppm and 200 ppm.

Testing 1, 2, 3

When you’re treating your pond with a product like Pond Logic Algae Defense, be sure to use a water hardness test kit, like Laguna Quick Dip Multi-Test Strips.

Carbonate hardness is measured in degrees (KH) or in parts per million (ppm). Because the water hardness test kit will give its results in degrees, you’ll need to convert your findings from KH to ppm to determine whether the levels in your lake are safe for your fish. Use this formula to figure it out: 1 KH = 17.848 ppm. So if your test kit reads 5 KH, you would multiply 5 times 17.848, which equals 89.24 ppm.

POND TALK: How often do you test your pond or lake’s carbonate hardness level?