I watched the English version of FM:s explanation how the Bolus method work and why it is a good policy to use NaHCO3 (bi-carbonate) instead for kalkwater or Na2CO3. among other things - it shows that adding Na2CO3 cause flocculation of a white stuff that among other thing says to affect the onset of Old Tank syndrome. First of all - the precipitate consists of CaCO3 - limestone and sometimes calcium phosphate - and how that can affect an onset of Old Tank Syndrome is beyond both my pay grade and my intellect.
But - yes doing the addition as shown in the video will precipitate limestone but no one dose either lime water or Na2CO3 this way - just because of this behaviour, Dosing of both lime water and Na2CO3 is done slowly in small portions in places with high throughput of water. hence either the pH spike or the concentration will cause ant precipitation. I dose my Core 7: 3a+b in my return apartment in the sump. It has a flow trough of around 30 l/min and an internal circulation of around the same amounts. I dose 1 ml/min in these stormy water - this 1 ml is diluted in around 60 L of water immediately as it hits the surface of the water. There is no chance of precipitation and if it should happen - the precipitation is to at least 99 % pure limestone (IMO).
In my world, it doesn't matter if you add pure sodium bicarbonate (NaHCO3) or sodium carbonate (Na2CO3). If you add NaHCO3 to water with a pH higher than about 8.15, a small part of the HCO3 ion will be transformed into an H ion + a CO3 ion. Slight pH reduction. If the pH is below about 8.15, the bicarbonate ion takes up a hydrogen ion and forms carbonic acid - pH increase. If you add Na2CO3 - at our pH - most of it will form HCO3 in the water by taking up a hydrogen ion to the carbonate ion - the pH is raised.
The formula below describes the equilibrium equation for our main buffer system. If any of the substances here (or the ions) (CO2, H2CO3, HCO3 and CO3) are artificially added then the equilibrium will shift with changes in pH as a result. The HCO3 ion is the balance point in the equilibrium. Adding it can depending on the starting pH either a lowering or raising of pH, Added of the substances on the right (CO3) always leads to an increase and addition of the substances on the left always causes a decrease. To make things even more difficult, you can also affect the basic pH (and thus the balance in formula) using external hydrogen ions or external hydroxide ions.
But the bottom line is that it doesn't matter if we use bicarbonate, carbonate or hydroxide ions to increase the alkalinity (In the pH levels present and normal in an aquarium) - most of the addition remains as bicarbonate (if bicarbonate is added) or is converted to bicarbonate if any of the others are added
There i two more things that has been mentioned as positive with the method. A pH increase with time and an alkalinity plateau will be formed. Calcification lower the pH. CO3 ions is on of the building bricks in the calcification. If one ion is removed from the system - around one bicarbonate ion will be transformed in to new CO3 ions in the system - loosing one hydrogen ion - pH depletion will follow - not a pH increase. I have problems with this increase of pH. The same with the plateau - where is it coming from?
The only times I have seen a alkalinity plateau and an increase in pH is when my calcification not works as it should....
Sincerely Lasse