Andy Pierce
Member
Filed under "DON'T TRY THIS AT HOME KIDS", for about six months now I've been having a play at using hydrochloric acid (HCl) to both permanently decrease water alkalinity and to temporarily increase soluble CO2 levels in the low-tech Shrimphaus setup (detailed comments here). In principle, this is pretty straightforward... water in Cambridgeshire (and London) is very hard with saturated levels of bicarbonate. You've got more CO2 in the water than you might ever need for plants but it's all locked away in the form of bicarbonate. The high alkalinity keeps the pH high and potentially isn't enjoyed by plants (and maybe animals) giving a "liquid rock" type of setup. Adding acid addresses both issues in a hard water, low tech setup: lack of soluble CO2 and high alkalinity.
The chemistry is interesting. The first thing that happens is the excess protons from the acid distribute across protonatable species: bicarbonate and carbonate. This reaction happens instantaneously as the HCl distributes through the water column. The chemical reaction that follows converting carbonic acid and bicarbonate into CO2 is not instantaneous, but is essentially complete within 1 second, giving a temporary CO2 boost. After the chemical reaction is completed, the boosted CO2 will gradually decrease over a course of hours due to outgassing to atmosphere and to consumption by plants. This gradual loss of carbonic acid also increases the pH. At the end of the whole process, alkalinity is permanently reduced, CO2 was temporarily increased for several hours, and the overall pH winds up slightly lower than where you started. Simulated reaction kinetics timecourses (which actually reflect real world experience pretty well as measured by pH) are shown below with details of what happens during the first second and then the overall picture. These simulations are for a tank with starting alkalinity of 290 ppm CaCO3, a starting pH of 8.5, and HCl added to 0.4 mM final concentration. CO2 equilibration to atmosphere is modelled here with a half-life of 1 hour.
I have been adding HCl from a 1M stock solution to a final concentration of 0.4 mM, twice a day, at the start of each split-lighting daily photoperiod. Each time this happens, the KH is permanently reduced by about 20 ppm CaCO3 (a little more than 1 dKH). The temporarily increased CO2 is definitely being taken up by plants, because the rate of consumption (measured by pH) is higher if the HCl is added with the lights on than with the lights off. Does it actually do any meaningful good? Very difficult to say. If done carefully, there are no obviously negative impacts on shrimp or snails (there are no fish in this aquarium).
I am not advocating this method - there are potentially catastrophic consequences if done incorrectly, e.g. if you actually drop the alkalinity so low that you run out of bicarbonate. The alternative of course would be high-tech CO2 injection (I do this on my other aquarium - note that CO2 injection can also potentially be catastrophic if done incorrectly) and using remineralised RODI water. If you just wanted the reduced alkalinity, you could use HCl to treat water you're going to use for a water change to get the alkalinity where you wanted it to be before putting it in the aquarium - this would be a less risky approach and does sort out the KH, but would miss out on the CO2 piece.
The chemistry is interesting. The first thing that happens is the excess protons from the acid distribute across protonatable species: bicarbonate and carbonate. This reaction happens instantaneously as the HCl distributes through the water column. The chemical reaction that follows converting carbonic acid and bicarbonate into CO2 is not instantaneous, but is essentially complete within 1 second, giving a temporary CO2 boost. After the chemical reaction is completed, the boosted CO2 will gradually decrease over a course of hours due to outgassing to atmosphere and to consumption by plants. This gradual loss of carbonic acid also increases the pH. At the end of the whole process, alkalinity is permanently reduced, CO2 was temporarily increased for several hours, and the overall pH winds up slightly lower than where you started. Simulated reaction kinetics timecourses (which actually reflect real world experience pretty well as measured by pH) are shown below with details of what happens during the first second and then the overall picture. These simulations are for a tank with starting alkalinity of 290 ppm CaCO3, a starting pH of 8.5, and HCl added to 0.4 mM final concentration. CO2 equilibration to atmosphere is modelled here with a half-life of 1 hour.
I have been adding HCl from a 1M stock solution to a final concentration of 0.4 mM, twice a day, at the start of each split-lighting daily photoperiod. Each time this happens, the KH is permanently reduced by about 20 ppm CaCO3 (a little more than 1 dKH). The temporarily increased CO2 is definitely being taken up by plants, because the rate of consumption (measured by pH) is higher if the HCl is added with the lights on than with the lights off. Does it actually do any meaningful good? Very difficult to say. If done carefully, there are no obviously negative impacts on shrimp or snails (there are no fish in this aquarium).
I am not advocating this method - there are potentially catastrophic consequences if done incorrectly, e.g. if you actually drop the alkalinity so low that you run out of bicarbonate. The alternative of course would be high-tech CO2 injection (I do this on my other aquarium - note that CO2 injection can also potentially be catastrophic if done incorrectly) and using remineralised RODI water. If you just wanted the reduced alkalinity, you could use HCl to treat water you're going to use for a water change to get the alkalinity where you wanted it to be before putting it in the aquarium - this would be a less risky approach and does sort out the KH, but would miss out on the CO2 piece.
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