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Carbonate Hardness & bio filtration

Discussion in 'General Plant Topics' started by MrThatcher, Nov 15, 2011.

  1. MrThatcher

    MrThatcher Junior Poster

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    New here so please forgive any naivety!

    I have a small (22l) fairly heavily planted tank which runs at pH 6.4, GH 7 and KH 0-1 (API reagent test, 1 drop turns yellow). The tank has inhabitants (fish & shrimp) and all are very healthy with no fish losses since I setup the tank 8 months ago. I do about 10-15% wc per week and the water added has a higher pH, GH & KH.

    I have read that @
     
  2. nipat

    nipat Guru Class Expert

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  3. MrThatcher

    MrThatcher Junior Poster

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    Thanks nipat. I know that at low pH levels the nitrifying bacteria start to die off but my pH remains pretty stable at 6.4. I would quite like to maintain the low pH, KH and GH as it benefits my fish & plants but I was just concerned that the very low KH is a bit of a time bomb in terms of bio filtration.
     
  4. nipat

    nipat Guru Class Expert

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    I don't know for sure if the bacteria will really die. Too many contradicted information in the web
    have made me a non-believer. I will trust only when I can prove it myself. But KH 4 doesn't hurt. :)
     
  5. Cyclesafe

    Cyclesafe Guru Class Expert

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    The nitrogen cycle converts 1 ppm (by weight) NH3 to 2.7 ppm NO2- (using 25.1 ppm HCO3-), which is then converted to 3.6 ppm NO3- (using another 7.2 ppm HCO3-). The total conversion of 1 ppm of NH3 to NO3- consumes 1.8 KH.

    NH3 + H2O = NH4+ + OH-
    NH4+ + 2H2O = NO2- + 8H+ (enzymatic reaction)
    OH- + H+ = H2O
    7H+ + 7HCO3- = 7H2O + 7CO2

    NO2- + H2O = NO3- + 2H+ (enzymatic reaction)
    2H+ + 2HCO3- = 2H2O + 2CO2

    This makes sense to me, but it's been 35 years since my last chemistry class. Bear in mind that plants will use up a portion of the ammonia, ammonium, and nitrite before the nitrogen cycle bacteria get to it. I can confirm that in my tanks, KH drops as does pH (CO2 solubolized as carbonic acid) as I delay water changes.
     
  6. MrThatcher

    MrThatcher Junior Poster

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    So basically the conversion of 1ppm of NH3 > NO3 consumes 1.8ppm KH?? But at what rate does this occur and how do you measure quantity of NH3 being converted, by NO3 measurement?
     
  7. Cyclesafe

    Cyclesafe Guru Class Expert

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    Not 1.8 ppm, 1.8 KH units. 1 KH unit = 17.8 ppm NaHCO3 equivalent. The rate depends on the efficiency of your bacteria. 1 ppm of introduced NH4OH (NH3 in water solution) is "almost immediately" processed to NO2 then NO3 by the bacteria in a mature biofilter - that's if the plants don't get it first. You'll not be able to measure NO2 with a test kit because it won't be able to build up to measureable levels before it is, in its turn, converted to NO3. I played around with this when I last established from scratch a biofilter using aqueous ammonia. It took 6 weeks to measure any NO3, but after the bacteria built up conversion of the ammonia was very rapid. But bear in mind that at the time I was looking to establish a biofilter for a fish only tank - not rigorously study the nitrogen cycle!
     
  8. MrThatcher

    MrThatcher Junior Poster

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    Thanks, that seems like an awful lot of KH would get used up very quickly converting NH3 in a well stocked tank.

    Obviously at a lower pH NH4+ is more prominent than NH3 and plants prefer NH4+ so maybe this is where most of the TAN gets taken up instead of getting converted by bacteria, which in turn would reduce KH consumption.

    Has anybody else had experience of running a stocked & planted tank with literally 0 KH?
     
  9. Cyclesafe

    Cyclesafe Guru Class Expert

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    Yes, it's true. My KH would drop 2-4 units per week in a fish-only 120 gallon tank (23x3" tropheus).

    I even attempted a nitrogen balance....

    On food labels, protein % is shown as nitrogen multiplied by 6.25, since the average nitrogen content of protein is about 16%. So, New Life Spectrum is 34% crude protein, so it's 5.44% N. Since NO3- is 22.6% N, each gram of NLS consumed and ultimately excreted by fish (assuming no weight increase in the fish) goes to produce 0.226g of NO3-. In a 120 gallon (454250 ml) tank, that's 0.50 ppm, consuming 0.50x32.3/3.6 = 4.48 ppm of HCO3- (0.25 KH units).

    I fed 10-20 grams of NLS per week (via an Eheim Auto Feeder), corresponding (as described above) to a loss of 2.5 to 5.0 KH units per week - reasonably correlating to 5 weekly KH observations measured with a calibrated LaMotte KH Alkalinity test kit. Nitrate was measured also, but even the LaMotte kit I have is not sensitive / reliable enough to make conclusions on this level.

    Although my experimental results seemed to validate the theory, by no means would I advocate that this is science. Being lucky is better than being smart. I do think, however, that one can conclude that KH is indeed consumed by the nitrogen cycle. When HCO3- is no longer available, it would follow then that pH would continue to drop, eventually affecting the efficiency of the biofilter. Good thing we have plants!
     
  10. MrThatcher

    MrThatcher Junior Poster

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    So seeing as my pH remains pretty stable (0.1-0.2 drop) over the course of a week after a w/c, I needn't worry too much that my KH is low?
     
  11. Cyclesafe

    Cyclesafe Guru Class Expert

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    http://www.dataguru.org/misc/aquarium/calKH.asp

    This calculator might be of interest to you...

    Changing water is key. Natural soft water environs can be as low as pH 5, but whether this is this due to dissolved CO2 (carbonic acid) or due to a dissociated stronger acid is the issue - not pH. I think that low pH that is the result of dissolved CO2 is no big deal - especially if there is plenty oxygen available for the fish to breathe and for the plants to respire at night.

    The issue about KH is that when injecting CO2 there is less buffering capacity when KH is low. This makes it easier to over-shoot and kill your fish. Hence the recommendation for KH 4 or so.

    The best indicator that you need not worry is that everything continues well for you. If you increase stocking levels, however, you might want to add a little baking soda to your water as insurance.
     
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