Who Gives A Fish Forums

I'm pretty new to water chemistry and I've never tested the GH and KH of my tank water before. I bought a test kit to find out my tank's GH and I've also got the results from my local water company, which are in Hardness Clark. I used an online calculator to convert the test kit results into Hardness Clark so I could compare them easily.

My test kit results are: KH - 71.2 ppm and GH - 106.8 mg/L.

WATER FROM TAP





TANK WATER





I used the Aquarium Test Kit from NT Labs.

KH is basically a measure of how well the water in my tank can handle changes in pH. GH is a measure of how many minerals are dissolved in the water.

I'm not sure if I've done the conversion correctly though - has anyone else done this before?

I've taken a look at your results, and it seems like you've got 71.2 ppm for KH and 106.8 mg/L for GH. Thing is, ppm and mg/L are essentially the same, so you can directly compare those values. Converting them to Clarke units, your KH would be around 5 and your GH around 7.5. We usually express these values in German degrees, so your KH would be 4 and GH would be 6. For reference, 1 German degree is equivalent to 17.848 ppm.

sidthy wrote: ↑Thu Jan 02, 2025 1:05 pm I've taken a look at your results, and it seems like you've got 71.2 ppm for KH and 106.8 mg/L for GH. Thing is, ppm and mg/L are essentially the same, so you can directly compare those values. Converting them to Clarke units, your KH would be around 5 and your GH around 7.5. We usually express these values in German degrees, so your KH would be 4 and GH would be 6. For reference, 1 German degree is equivalent to 17.848 ppm.

Thanks for explaining that. I now see why my chemistry skills are lacking, I guess that's what I get for trying to tackle water chemistry without a solid foundation.

Thanks for the explanation, sidthy. I can see now why chemistry wasn't my strong suit, but I guess that's not too surprising given my lack of knowledge in water chemistry to begin with.

GH can be a real challenge, especially when compared to litres/US gallons/Imperial gallons for volume. In fish keeping, we mainly use two units, but there are many others that can be confusing. Some units are so similar they're essentially the same.

For instance, mg/l calcium carbonate and ppm are interchangeable. Some UK water companies prefer mg/litre calcium, while others might use mg/l calcium oxide. Then there's German degrees, also known as dH or dGH, and French degrees, which have different names but are the same unit. Clarke or English degrees are also the same, just with different names. The US uses Grains per gallon, which is almost identical to dH/German degrees, but with a slight difference.

I've also come across mmol/litre and mval/litre in my reading, although I've never seen them used in practice.

What's interesting is that units like mg/l calcium carbonate, calcium, and calcium oxide don't actually represent the amount of those substances. Instead, they represent what the number would be if all the minerals that make up hardness were those substances. My son, who has a doctorate in chemistry and worked as an analyst for a water testing company, explained this to me. He helped me understand that these units are more like a representation of the overall hardness.

In fish keeping, we mainly use ppm/mg/l calcium carbonate and dH/German degrees.