I was expecting that around 13:00 pm I would get the pH drop by 1, but it never went down below 6.2 throughout. Since I have started this aquarium around 50 days ago and using Tropica Aquarium Soil, could the soil be preventing the pH from dropping lower than 6.2 ?
Any insights based on the pH profiling above ?
Hi Mel,
OK, well, first of all, it's really important to understand the reason we are using a pH drop. I feel that a lot of folks don't understand why we do it. We just tell them they should do it, and they follow suit, not always understanding the mathematics of the method. As you probably know (or perhaps were told) if you measure the pH of the tank water and plug that reading into the infamous pH/KH/CO2 chart you will almost always get the wrong CO2 value because the tank water is affected not only by the Carbonic acid produced by the CO2, but the pH is driven even lower due to the organic acids present in the water. So folks who are unaware of this phenomenon will always read a higher CO2 value than there actually is in the water.
So we take a pH reading at the start and that gives us a pH value composed of a [pre-CO2 induced acidity + Organic acidity] then we take a reading later in the day to get a [Co2 induced acidity + Organic acidity].
When we say find the "pH drop" what we are really saying mathematically is to subtract the CO2 induced reading from the pre-CO2 induced. A geek would be write this as a simple equation:
[pre-CO2 induced acidity + Organic acidity] - [Co2 induced acidity + Organic acidity]
Which then becomes:
pre-CO2 induced acidity + Organic acidity - Co2 induced acidity - Organic acidity
The expression "Organic acidity" cancels because there is one with a plus (+) and one with a minus (-).
So the equation now reduces to:
pre-CO2 induced acidity - Co2 induced acidity
This is exactly what we are trying to do. Since BOTH pre and post CO2 reading are equally affected by the same organic acid, taking the DIFFERENCE between the two readings effectively eliminates the organic acid contribution from the equation. We can be confident that over the span of a few hours the organic acid content of the tank water will not appreciably change. We assume it will have the same value and therefore subtracting one pH reading from the other, even though we do not know exactly the quantity of the organic contribution, will cancel the organic acid content.
Now, in the ideal world we'd be able to go to the chart with your numbers:
pre-CO2 is 6.9 and post CO2 is 6.2
For any KH we would plug 6.9 in the chart and the chart would returns a fictitious CO2 value.
Then we would plug 6.2 in the chart and it would returns another fictitious CO2 value. Ideally, we would subtract one from the other and this would tell us our true CO2 value - except, now we get to the second problem with the pH/KH/CO2 chart. Not only does it assume that the acidity of the water is caused ONLY by CO2, but it also assumes the KH of the water is ONLY cause by carbonate. As we should know, our KH test kit is NOT a KH test kit. It is an ALKALINITY test kit and the reading it returns is actually due to many possible components not named carbonate or bicarbonate.
The third problem with the chart is that the data is not linear, it's logarithmic because pH itself is logarithmic. We've kind of figured out that a drop of about 1 unit will give you about 30-40ppm but these are just basic numbers. Everyone's alkalinity composition differs as does their tank acid contribution. This is why I'm always warning people to NOT get caught up in chasing numbers as if they were the Holy Grail. At best they are rules of thumb. You still have to monitor your fish and plants.
This was a really long winded way of saying "No it has nothing to do with you're soil and everything to do with your injection rate and flow/distribution." In any case, just live with your 6.2 for the moment and when you have a lot of time to experiment with the injection rate and are able to monitor your fish make the adjustments as needed. If your plants are happy with this configuration then leave it be. If you are experiencing CO2 related algae or CO2 related faults then you might want to think about trying to optimize further. Having said that, it should not take 4 hours to drop the pH 0.7 units and as I mentioned, that is most likely due to some combination of injection rate, diffusion method, excessive surface agitation and flow/distribution.
Continue to monitor with your new probe. Keep the tip moistened and calibrate frequently. Every time you make a change to your setup repeat the exercise and record the data so that you can analyze at a later date. Avoid the pH/KH/CO2 charts as if it were the plague.
Cheers,