# Potassium dosing



## Lord Nibbler (Dec 22, 2005)

I have two 37-gallon aquariums: one moderately/heavily planted and one lightly/moderately planted. In the heavier planted one, the Ludwigia "tropicana" gets the little pinprick holes (I assume from K) and in the other aquarium, the Anubias heterenophyllia (or however it is spelled) gets them too. I dose with potassium nitrate a couple times a week (about 1/2 a teaspoon) and the nitrates typically hang around 25ppm with a few small water changes a week.

I was wondering if there was a way to directly measure potassium. I'm assuming its low, but how much should be added to a 37 gallon aquarium a week? My tap is high in phosphates, so if anything I'd be adding potassium sulfate along with the trace, salts and KNO3.


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## bigstick120 (Mar 8, 2005)

The test kits that I have seen for K test in a low range, useless for us and around 30-40 bucks. Pinholes is usually a sign of low K, if you have some on hand
KH2SO4 I would dose an extra 1/4-1/2 tsp when you add the KNO3. I dont think there are any side effects to an overdose of K.


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## wiste (Feb 10, 2006)

> The test kits that I have seen for K test in a low range, useless for us and around 30-40 bucks.


Given a source of pure water, a low range test kit can be used to test higher levels by diluting the test solution.


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## werner (Jul 6, 2006)

These guys have a potassium test kit- is this what you're referring to? https://ssl.perfora.net/www.aquariu...nid=15450749858e2d8/shopdata/index.shopscript


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## Salt (Apr 5, 2005)

werner said:


> These guys have a potassium test kit- is this what you're referring to? https://ssl.perfora.net/www.aquariu...nid=15450749858e2d8/shopdata/index.shopscript


That kit only tests levels between 1 and 2 ppm. Not very useful.

Lamotte makes a kit that works very well. You need a bright light and white surface to hold the tube over (it's a turbidity test). You also need distilled or RO/DI water. It tests between 6 - 50 ppm.

Here's a PDF file of the kit's instructions.

I've not seen a site with a nice "add to cart" button for these kits. I purchased mine from Clarkson Lab. I had to e-mail them my order.


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## Edward (May 25, 2004)

PF solution gives you the right amount of Potassium. No need to test it.


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## wiste (Feb 10, 2006)

> I was wondering if there was a way to directly measure potassium.


As mentioned before, if you have a low range test kit you can dilute the test solution to read higher values. For example you can prepare a 10% solution of the test solution. Take 10% of water to be tested and 90% of distilled/RO water. If you use a 10% solution and this reads 1ppm, then this equates to 10ppm in the aquarium water being tested.

It is desirable to calibrate your test kit. Additionally ensure that your pure water (distilled/RO) water yields 0 ppm of potassium. Use of pure water and precision in measurements is critical to your results.

To calibrate, mix up a known concentration of potassium. For example, use the pure water and add an amount of potassium to bring the level up to 1pmm. If your test shows the expected level, then the test is good. If the level is not as expected then readings must be adjusted.

One assumption is that you can use the ratio of the actual/expected to convert future readings.
If the test shows 1ppm for a 2ppm concentration, then you must adjust the results for the aquarium water being tested in a similar manner. Multiply future readings by this ratio. This assumes that all readings will be off by this factor.

This may not be the case. You could verify the ratio by taking a lower and higher reading in the range of measurements. If they are not off by the same ratio you could assume the ratio is linear. Then you could take a third reading and verify the relationship does change linearly. If linear you then use the point slope formula to convert the reading. If not linear you could find an equation to relate the readings but this would be overkill. Using the ratio is probably good enough for the hobbyist.

How to prepare a test solution:
The amount to bring the solution to 1ppm use the conversion of mg to 1 mg/L = 1 ppm. The mg of potassium added for each mg of potassium sulfate is determined by the percentage of potassium in potassium sulfate. You have to use the formula for potassium sulfate and the atomic weight of potassium, sulfur and oxygen.
The formula for potassium sulfate is K2SO4
atomic weights
Potassium is 39, oxygen is 16 and sulfur is 32
K2S04 is 2*39+4*16+32=174

Percent potassium = 100*2*39/174 = 44.8%
2.32 mg of K2S04 added to one liter would yield a 1 ppm calibration solution for potassium.

As mentioned above, following PPS would ensure adequate potassium levels without testing potassium.


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## Lord Nibbler (Dec 22, 2005)

I think it would actually be easier just to buy some potassium sulfate and see if it fixes the problem. Thanks for the info!


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