# Biogenic Decalcification



## AverageWhiteBloke (3 Feb 2018)

I could be wrong at the moment I would have to test more but if plants are co2 limited and extracting carbon via carbonates in the water, could they remove enough carbonates to lower PH through the lighting period?

The reason I ask is my tank was running KH limited, only just realised it. I've added some crushed coral (I Think) to raise KH, its a non co2 tank btw. I also have a fair amount of leaf litter which will reduce PH slightly or quite significantly I hazard a guess in no KH water! However, yesterday having adding crushed coral the PH seems to have stabilised this morning. Tests again after lighting period show the ph has actually lowered since this morning.

Question is could the plants have sucked the carbonates out during growth to lower the PH? Everything seems to be doing the exact opposite of what I was expecting.


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## dw1305 (3 Feb 2018)

Hi all, 





AverageWhiteBloke said:


> The reason I ask is my tank was running KH limited, only just realised it. I've added some crushed coral (I Think) to raise KH, its a non co2 tank btw.


The plants will deplete the available CO2, but when you have some dKH, you don't change the amount of dissolved inorganic carbon (DIC), you just change the form it is in.





With no carbonate reserve in the water you don't have the bicarbonate (HCO3-) and carbonate (CO4--) ions, but when the plants are photosynthesising they will produce oxygen, and oxygen is a base. When they stop photosynthesising the CO2 (from respiration) level rises and the pH falls. Because you don't have any reserve of carbonates the pH fall will be large.

This is from Diana Walstad's "Ecology of the Planted Aquarium"





> Daily variations of the water parameters are rarely – if ever – taken into account. Data collected in a freshwater lake (Star Lake, VT) with a very low alkalinity showed a diurnal pH fluctuation beyond the imagination of most hobbyists. Thus, the pH at 10 am was measured at 5.7 (strongly acidic), 9.6 at noon (strongly alkaline), 8.3 at 2 pm (moderately alkaline) and finally, 6.4 at 4 pm (slightly / moderately acidic). Readings were taken at a 0.5M depth. The fluctuation observed was due to the low KH value of the water (something reported for the Amazon river, too) and the presence of large amounts of phytoplankton. Under the circumstance it is extremely difficult, if not impossible, to figure what is the “right” pH for any form of aquatic life collected in that lake and which tank could cope with this kind of fluctuation. The low – high points of the day differ by 4 pH points, which means that the concentration of H+ in the morning is 10.000 times higher than at noon, while this change takes place in just two hours. It goes without saying that this pH swing cannot be observed in an aquarium only because we cannot reproduce the amount of light which falls in the Lake. In any case, if somebody reported that a suitable pH for aquatic life collected in this lake should range daily from 5.7 – 9.6 most hobbyists would think it was a typo.


cheers Darrel


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## AverageWhiteBloke (3 Feb 2018)

dw1305 said:


> Hi all, The plants will deplete the available CO2, but when you have some dKH, you don't change the amount of dissolved inorganic carbon (DIC), you just change the form it is in.
> 
> 
> 
> ...


Still digesting that Darrel, I do have the Walsted book on ereader but I guess I haven't got to that bit yet. I find there's a lot to take in with that book so I tend to go over the same bit a few times until I understand  it, I suffer from information over load haha.

Are you saying that the co2  and PH will go down after lights off? I'll put some flesh on the bones.

My tapwater has very little KH, generally 2nd drop on a test kit although it's borderline wanting to go on the first drop. Considering KH tests tend to measure higher than actual there's not much kh in I guess. With adding of fish etc I've been doing small scale wc's  every couple of days or whatever but this is the first couple of weeks I've been doing nothing but topping up to see how the tank goes on a normal week.

My tapwater has a PH of 7.2 / 7.3 so ph isn't something I fuss over. Just happened to test the water on Friday and found my ph was 6.0 with both digital and solution kit. Tested KH in tank and changed 1st drop on a 10ml solution do less than half a degree. I added about a teaspoon of the coral gravel directly into tank. Not sure what it is, I got it from the lfs in a bag some while ago. 

Anyway, last night at lights just off ph was 6.0, today before lights knocked on ph was 6.47 so I assumed things were evening out and the coral was probably neutralising acid from the leaves while dissolving some kh into the water. Then tonight about 10 mins before lights off I was getting 6.0-5.9 

I would have expected the lower ph to be during the dark hours no? Unless the leaf litter is still overwhelming the coral? Just thought I would get a higher reading at the peak of photosynthesis. 

Sent from my STH100-2 using Tapatalk


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## dw1305 (3 Feb 2018)

Hi all,





AverageWhiteBloke said:


> Are you saying that the co2 and PH will go down after lights off?


The CO2 level will rise (the bioload is still respiring, but you don't have any photosynthesis to use the CO2) and the pH fall (you have more CO2, and a small proportion of this will go into solution as H2CO3)  and less oxygen (you don't have any photosynthesis to produce oxygen). 





AverageWhiteBloke said:


> Anyway, last night at lights just off ph was 6.0, today before lights knocked on ph was 6.47 so I assumed things were evening out and the coral was probably neutralising acid from the leaves while dissolving some kh into the water. Then tonight about 10 mins before lights off I was getting 6.0-5.9


I'd just ignore pH, as you get towards pure H2O it is a less and less useful measurement.





AverageWhiteBloke said:


> Tested KH in tank and changed 1st drop on a 10ml solution do less than half a degree.


Any addition of acids (H+ ion donor) will reduce the dKH.  If you have very low levels of dKH, even a small addition of H+ ions (like the humic acids from leaf litter) will reduce the carbonate hardness (the HCO3- and/or CO3--, is converted to CO2). 





AverageWhiteBloke said:


> Unless the leaf litter is still overwhelming the coral?


It will take a while for the coral gravel (aragonite form of CaCO3) to raise the dKH, how quickly it dissolves depends the amount of acids in the water. Carbonates are insoluble in water, but soluble in weak acids, which is why you get shell reefs form in (the very carbonate rich) Lake Tanganyika, and coral reefs in the (carbonate rich) oceans.

The acid dKH reaction is the the same reaction you get when you pour <" HCl onto limestone">, the bubbles you get are the evolved CO2.

cheers Darrel


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## AverageWhiteBloke (4 Feb 2018)

Thanks for taking the time to explain that to me Darrel, up to press any setup I've done has had a fair amount of seiryu stone in it so KH has never been an issue as I've always been aware of how soft my water is.



dw1305 said:


> Carbonates are insoluble in water, but soluble in weak acids



Am I right in assuming that it will still raise KH though in soft acidic water and dissolve into the column?

Had a bit of a flashback to a shrimp tank I set up many moons ago, again it was a tank with just tapwater and no stone to mineralise it. I can't find the post now but I had a total disaster where I already had a fair amount of leaf litter and added a Catappa log for the shrimp to hide in, next day some shrimp had died and got stuck on the filter intake which fouled the water and I lost my shrimp in biblical proportions. At the time the blame was put on the ebay log and people suspected possible contaminants on it. Looking back I probably crashed the system without realising, it was also getting a small amount of co2 diffused in as well. I bought the bag of coral back then thinking it would help with the shrimp moulting.


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## AverageWhiteBloke (4 Feb 2018)

https://ukaps.org/forum/threads/rcs-losses-after-filter-change.26463/   Found it.


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## dw1305 (5 Feb 2018)

Hi all,





AverageWhiteBloke said:


> Am I right in assuming that it will still raise KH though in soft acidic water and dissolve into the column?


Yes it will, this is how you get hard ground water and caves formed where the geology is limestone.





AverageWhiteBloke said:


> Looking back I probably crashed the system without realising,


I've read through the post, and I think you are probably right. Cherry shrimps definitely don't do well in <"very soft water">, but because of the speed of the deaths I think the that the most likely option is that the lowering in pH caused changes in water chemistry, possibly because formerly insoluble metals went into solution.

cheers Darrel


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## AverageWhiteBloke (5 Feb 2018)

Thanks Darrel, looks like KH is something I need to keep an eye on in these no stone, leaf littered setups. The coral gravel looks a bit weird being white on a reddish brown substrate. I just dumped it in at the back where you don't see it. I might put the hammer through some of my Seiryu stone and scape a couple of pieces in just to help with the buffer and look a bit more pleasing on the eye. Been doing water changes once per week at 30%, the tapwater has some KH but obviously not enough if the tank is being left for any length of time, in my quest for a blackwater type setup with tea stained water I have say half dozen Alder cones and Oak leaves and a medium sized Almond leaf in there. I'll try and get the KH raised to around 2 just for sufficient buffer against them.


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