Carbonate Hardness & bio filtration

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.

 

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.

 

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!

 

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!

 

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.