Fish Tank pH and kH

kh carbonate hardness test kit 0132 Aquatic Veterinary Services

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It’s time for everyone’s favorite topic: water quality! Today we will be learning about your fish tank pH and supporting kH, also known as alkalinity.

What is pH?

Fish tank pH is a measure of the hydrogen ion concentration in a solution. Hydrogen ions play an important role in many chemical and biological functions. Some fish have a wide range of pH tolerance, while others have a very strict range.

How do I test my fish tank pH?

Many of our clients purchase “master” test kits which include: ammonia, nitrite, nitrate, low and high pH. The low and high range pH can sometimes be confusing for fish tank owners. Essentially, if your low range pH is at the very high end of its range, switch to the high end. If your high end pH is at the very low end of its range, switch to the low end. You may be interested in an electronic probe instead; just be sure to keep it calibrated! This is a great collection of parameters to start with, but what about kH?

What is kH?

For those of you unfamiliar with kH, kH measures the buffering capacity of your water. Buffers bind to free hydrogen ions (H+) and keep them out of solution. Since your pH is a direct measure of [H+] in your system, kH directly impacts pH. And remember that high [H+] = more acidic and low [H+] = more basic.

(For those of you who are confusing this with gH, or total hardness: gH measures the amount of calcium and magnesium in your water. Although this may be a component of your kH, they are separate parameters entirely.)

So, if your kH levels are too low, all the H+ your fish discharge due to metabolic processes can build up and severely alter your fish tank pH. Adequate kH levels will keep your pH consistent throughout the day, regardless of what your fish and filters are doing. Knowing your tanks kH is an essential component to any fish keeper’s database. There are simple tests available that will make your testing a true “master kit.”

What is the best level of kH for my fish tank?

A kH value of >50 mg/L is adequate, but >100 mg/L is better. 

If your tank kH is low, test your source water kH. Some city and well systems have low kH coming in and will need buffers to be manually added to your system to maintain adequate levels. Keep an eye on your pH when manually adding buffers in order to make sure your pH will not change too much. Slow and steady is the goal for any pH changes.

And you may have species, like some cichlids, who like a range between 50-100 mg/L. Do your research before you buy any fish to make sure you can provide them a comfortable home!

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9 thoughts on “Fish Tank pH and kH”

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  6. Avatar of Jennifer Drennan

    Hi there, very useful explanation! I am just starting out, currently cycling a planted tank, no fish yet. I have very soft water 0 kh and negligible gh and a pH around 6.8. How can I increase kh and gh without changing ingredients the pH? I’m finding lots of advice online but all of it increases pH. I’m terrified of adding fish and the low kh allows a drastic pH swing which kills them all, I’d feel terrible.
    Any advice would be most gratefully recieved
    Many thanks,

    1. Avatar of Dr. Jessie Sanders
      Dr. Jessie Sanders

      You will change the pH no matter what you use to increase your KH. This is normal, but needs to be done slowly. With any buffer that you use, it is critical that you test regularly and start with very small doses and titer up.

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