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Discussion Starter · #1 ·
Hello all; I'm going to return to planted tank hobby and i'm considering what fertilizing methodology to use. In the past i used very pure KHCO3 (not K2CO3 !) as a potassium source but when i added about 2 ppm of K daily into the tank many of my fish died. When i added the same amount of K from K2SO4 everything went OK. I repeated the experiment with KHCO3 some time later and it killed some fish again.
I suppose it was caused by pH swing which was due to alkalinity of KHCO3 but i'm not sure. If memory serves me correctly one day i increased CO2 level and fish were happy even with high dosage of KHCO3.
It seems that KHCO3 solution does't have high pH so it's very strange that it harmed my fish. Did anyone of you have such problems ?
 

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Hi kekon,

If you have very soft water with KH aroud 0 then adding KHC3 change KH slightly (exactly +0.15 KH for 2 ppm K) but pH can dop a lot for a moment if the CO2 level is small. Anyway I don't give this big chance. In my aquariums I have GH 3, KH 0, pH 6.3 and never had problems like yours. I used KHCO3 and K2CO3 somtimes even in 3.4 ppm K daily doses. Many times. Nothing happend in aquariums with RC shrimps and small fishes.

So I guess there are some bad impurities in your KHCO3. Even though you used "very pure" (does it mean clean for analisis?) it contains 99.9% of KHCO3. The rest 0.1% are impurities dependent of what was the method of obtaining chemicals. If this 0.1% contains for example something with NO2 it will kill a life in whole aquarium. Normally chemicals producer specify what kind of impurities it contains. Check it and buy another KHCO3 from different source.

Good luck and I hope that I helped you some.
Piotr
 

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Discussion Starter · #3 · (Edited)
I suspected my KHCO3 had some impurities so I bought some more with the highest purity (clean for analisys) available and the effect was the same. The label on the "Brighty K" (which contains KHCO3) bottle says to use it sparingly as it can kill shrimps.
It seems that the problem may be caused by too low CO2 which but i'm not sure.
If there is 0.1% of NO2 in KHCO3 it should not do anything bad in my opinion. Even with 30 ppm K 0.1% will be only 0.03 ppm NO2. Such level is negliglible.
 

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Yes, NO2 wasn't good example but knowing the kind of impurities you have is a good idea. Just to be sure.

I don't know if pH rise kills your shrimps or something else. Anyway your problems are rather connected with HCO3- ions. Directly or indirectly. Did you try K2CO3 in the same concentration of K? If not you should try. It left CO3-- ions so the reactions should be some different and maybe the final efect will be less deadly.

You could also try dose twice (or more) a day in smaller portions. This way checks if sudden change is a problem or the chemicals ppm/day.
 

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Discussion Starter · #5 ·
K2CO3 - no way !!! It must not be used in a planted tank. I know some people do but it is very basic and pH swings are much higher than in case of KHCO3.
I used it with very bad results. Not only did the water turned cloudy (CaCO3 was precipitated) but also all the plants were stunted. The pH of KHCO3 solution (50 g in 1 liter of water) is about 8.5. In case of K2CO3 it will be much higher.

I found an interesting thread about KHCO3:

http://www.plantedtank.net/forums/385504-post7.html
 

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Alkalinity of KHCO3 or K2CO3 water solution, for the equal ammount of K, is the same!

Adding K2CO3 works fine for me (the same KHCO3). Never have cloudy water. No problems with stunning growth. Tested on about 50 different plant species present in four aquariums with CO2 and with glutaraldehyde (all four have GH 3, KH 0).
But I have never used it more than 3.4ppm of K/day (for few days). Normal day dose for longer period was 1.7 ppm K/day.
I am not the only one using K2CO3. Look at http://www.theplantedtank.co.uk/RO.htm. He uses higher concentration in his remineralising agent.

Simple conclusion: we have quite different water chemistry in our tanks.
 

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Discussion Starter · #7 ·
Alkalinity of KHCO3 or K2CO3 water solution, for the equal ammount of K, is the same!
I don't think so. Some data from K2CO3 and KHCO3 data sheet (www.poch.com.pl):

K2CO3:

pH: about 11,5 - 12,5 (50 g/l H2O,20°C)

KHCO3:

pH: about 8,5 (50 g/l H2O,20°C)

I added the same amounts of K from K2CO3 and KHCO3. In case of KHCO3 i never had cloudy water but when i dosed K2CO3 i always noticed CaCO3 precipitation which is a clear sign that K2CO3 solution with the same amount of K has greater pH.
 

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My alkalinity (buffering capacity) understanding is like expressed here: http://faq.thekrib.com/begin-chem.html. It depends of -HCO3 and --CO3 ions ammount.

Go forward and make some test. Take 1liter of water from your aquarium add 5mg KHCO3 and another 1liter of the same water put in 3.5mg K2CO3. The K doses are about 2ppm. Their alkalinity or KH should also be the same. Check it. You can also check how pH changes etc. And all this without disturbing plants or fishes.
 

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Discussion Starter · #9 ·
Piotr Sroczyński;620889 said:
My alkalinity (buffering capacity) understanding is like expressed here: http://faq.thekrib.com/begin-chem.html. It depends of -HCO3 and --CO3 ions ammount.

Go forward and make some test. Take 1liter of water from your aquarium add 5mg KHCO3 and another 1liter of the same water put in 3.5mg K2CO3. The K doses are about 2ppm. Their alkalinity or KH should also be the same. Check it. You can also check how pH changes etc. And all this without disturbing plants or fishes.
I made 2 solutions and measured pH with electronic pH meter:

50 g KHCO3 in 1 liter: pH = 8.45
35 g K2CO3 in 1 liter: pH = 10.92

So, pH is not the same.
 

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I made 2 solutions and measured pH with electronic pH meter:

50 g KHCO3 in 1 liter: pH = 8.45
35 g K2CO3 in 1 liter: pH = 10.92

So, pH is not the same.
Some misunderstanding is here. I wrote that alkalinity is the same (this what we measure by KH test) not pH.

Why you tested pH for such strong solutions? You add to the tank 3.5mg K2HCO3 per 1 liter of water not 35g!
Concentration in tank will be 10,000 times smaller! Try check the pH after such dose and use water from your tank. Only such measure would give you some interesting information.
 

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Hi Kekon, in the tank where you had this issue with the K2CO3
What's the KH's value?
If the alkalinity isn't too low (something like KH=3°dKH or more) the pH fluctuation when you add the K2CO3 shouldn't be high. If the KH's value it's near to zero the pH fluctuation when you add the K2CO3 may be higher (and ok, Piotr haven't any issues even in this case)
 

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Discussion Starter · #13 ·
The KH was about 3 dKH. You're right that it could have been too low.
Most people confuse K2CO3 with KHCO3. When using K2CO3 i also noticed stunting in plants. It never happened with KHCO3.
 

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Sudden pH changes in a tank is what harms fish. Plants ignore that.
It's not that simple. When Co2 is used pH drops sometimes a lot. After big water change pH can suddenly change a lot. In most situations there is not visible harm to the fish.

How KHCO3 can be made less toxic in your tank?
Try: Take some aquarium water (1 l) dose KHCO3 to this water. Wait some time (20 minutes). Pour to the tank.
Or: Divide the KHCO3 dose on more smaller doses.
 

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The pH value determines the equilibrium between H2CO3 <-> HCO3- <-> CO3--
The issues with the K2CO3 solution could be caused by a temporary raise of the CO3-- concentration?
At the equilibrium both solutions that add the same amount of K+ (KHCO3 and K2CO3) bring at the same alkalinity so the equilibrium point it's the same (same concentrations of H2CO3, HCO3- and CO3--)
 

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Discussion Starter · #17 ·
I think i will buy KHCO3 from another supplier. Perhaps the one i used really had some impurities (despite the fact it was described as "clean for analisys") ?
Some people say plants utilize K much better from KHCO3 than from other sources.
 

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In the water:
KHCO3 --> K+ + HCO3-
K2CO3 --> 2*K+ + CO3--
K2SO4 --> 2*K+ + SO4--
KCl --> K+ + Cl-
So in all cases in the water there are the same K+ ion so for the plants, all these sources of potassium are the same.
The difference is that K2CO3 and KHCO3 raise the alkalinity, the KCl may bring to a Cl- build up and the K2SO4 may bring to a SO4-- build up. Generally the SO4-- build up don't causes any issues as long as the concentration of SO4-- don't becomes really high.
 

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At the water changes I use RO water and then I add KHCO3 to raise alkalinity and I add also KHCO3 during the week as a potassium source and I haven't any issues with fishes, shrimp or plants. (Or at least I have never recognised issues due to this)
In the past I used also K2SO4 as a potassium source and it also did the job.
 
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