Water chemistry question--carbonates?? - The Planted Tank Forum
 
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post #1 of 12 (permalink) Old 11-08-2012, 06:00 PM Thread Starter
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Water chemistry question--carbonates??

An unusual situation: I recently noticed a white chalky substance that has precipitated on my gravel and glass. Accompanying this was a drop in KH, so I am suspecting that something has caused my carbonates to precipitate. Does anyone know what may have caused this?

The only thing I can imagine is that it would have something to do with the plant fertilizers I am using: small amounts of dry macronutrients (Potassium Nitrate, Monopotassium Phospate, Potassium Sulfate) and traces. Could an imbalance or overdose of one of those caused this precipitation?
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post #2 of 12 (permalink) Old 11-08-2012, 06:27 PM
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What kind of plants do you have?

Some species are reported to perform biogenic decalcification whereby they can obtain carbon from molecules that contribute to kH, resulting in the phenomenon you have described.

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post #3 of 12 (permalink) Old 11-08-2012, 06:30 PM
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KH2PO4 dosed together with traces containing iron can result in the phosphorus and iron bonding and falling out of solution. Not sure about the color though.

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post #4 of 12 (permalink) Old 11-08-2012, 06:33 PM Thread Starter
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What kind of plants do you have?

Some species are reported to perform biogenic decalcification whereby they can obtain carbon from molecules that contribute to kH, resulting in the phenomenon you have described.

I have anacharis and a moss ball. I know anacharis can pull carbon from the carbonate, but it seems strange that this would happen all at once after the KH had been stable. Also, if the anacharis was consuming the carbon, what would be the substance that was precipitated--the calcium?
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post #5 of 12 (permalink) Old 11-08-2012, 06:35 PM Thread Starter
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KH2PO4 dosed together with traces containing iron can result in the phosphorus and iron bonding and falling out of solution. Not sure about the color though.
Interesting. I did overeagerly fertilize a few things together after doing a large water change the other day...I'm assuming that something happened as a result (and I learned my lesson and won't do it again ). But if phosphorous bound to iron, why would my KH have dropped?
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post #6 of 12 (permalink) Old 11-08-2012, 07:14 PM Thread Starter
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I have anacharis and a moss ball. I know anacharis can pull carbon from the carbonate, but it seems strange that this would happen all at once after the KH had been stable. Also, if the anacharis was consuming the carbon, what would be the substance that was precipitated--the calcium?
I found another guy's post on this forum in which he said adding too much iron caused iron carbonate to precipitate...I assume iron is included in my trace fertilizer.
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post #7 of 12 (permalink) Old 11-08-2012, 10:34 PM
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KH2PO4 dosed together with traces containing iron can result in the phosphorus and iron bonding and falling out of solution. Not sure about the color though.
yellowish white to cream colored.
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post #8 of 12 (permalink) Old 11-08-2012, 10:55 PM
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I found another guy's post on this forum in which he said adding too much iron caused iron carbonate to precipitate...I assume iron is included in my trace fertilizer.
Iron is in your trace fertilizer. You would really have to be dosing a lot to precipitate out FeCO3 with the iron in your trace unless your KH is pretty high. Usually a problem for people with a KH in the 10 to 12 range.

But it is possible and would correlate with your witnessed precipitate and your witnessed drop in KH.
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post #9 of 12 (permalink) Old 11-08-2012, 11:36 PM Thread Starter
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Iron is in your trace fertilizer. You would really have to be dosing a lot to precipitate out FeCO3 with the iron in your trace unless your KH is pretty high. Usually a problem for people with a KH in the 10 to 12 range.

But it is possible and would correlate with your witnessed precipitate and your witnessed drop in KH.

No, my KH is normally around 4 or 5. I've done a couple water changes to get the KH up a bit, and I'll see if it holds. It's possible I miscalculated when I mixed the trace, I'll re-mix before I dose again.
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post #10 of 12 (permalink) Old 11-09-2012, 12:23 AM
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When I was overdosing PO4 so much, I got what I thought it was
Calcium Phosphate as precipitation in CO2 reactor.

But the precipitation shouldn't cause KH to drop (I'm not sure here).
Since the Ca was from CaSO4 I dosed.
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post #11 of 12 (permalink) Old 11-09-2012, 01:22 AM Thread Starter
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OK all....thanks for all your advice. From a bit of my own research, I'm fairly sure that I am seeing the results of biogenic decalcification....so, how do I stop this from happening? Less nutrients? Inject CO2? Less light? Get rid of the anacharis?
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post #12 of 12 (permalink) Old 11-09-2012, 03:48 PM
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Add CO2.
Plants prefer CO2 as the easiest source of C. That you are seeing the results of them attempting to get C in other ways shows that they are having to work harder, that C in the form of CO2 is not available.
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