Hi all,
The hardness of the water (or its pH) shouldn't make any difference,
as long as you have 4dKH indicator solution in your drop checker.
When you have 4dKH solution + bromothymol blue indicator, you can use the experimentally defined equilibrium values between dKH ("bicarbonate" HCO3-) and CO2 (as "carbonic acid" H2CO3), to give you the level of CO2 in the tank, (
nb. only a very small proportion of CO2 becomes H2CO3, 99.8% remains as the dissolved gas).
We use bromothymol blue because it is a narrow range pH indicator, yellow at pH6, green at pH7 and blue at pH8. Table below.
Why we need to use 4dKH indicator solution in the drop checker
This is really important, if you had tank water with 20dKH it would naturally be at ~ pH7.8 (at the equilibrium value between HCO3- and atmospheric CO2 levels), but it is almost endlessly carbonate buffered, and if we then added enough CO2 to depress the pH to pH6.8 we would have
94ppm of CO2 and a lot of dead fish.
The Bouncy Castle analogy
If that doesn't make sense and I don't know if this helps, but I'll use the "Bouncy Castle" analogy where "compressed air" substitutes for "added CO2".
When we aren't actively pumping air into the bouncy castle via a compressor, it is deflated and the air pressure inside and outside are the same, this is analogous to a fish tank where we aren't adding CO2.
When we turn the compressor on air is pumped into the castle and it inflates, in the case of CO2, when we add CO2 the HCO3- ~ H2CO3 equilibrium is driven towards H2CO3, you now have an extra H+ ion, and acids are defined as H+ donors, so the pH falls (the water becomes more acid).
If our bouncy castle doesn't have any holes in it, we don't need to add much compressed air to keep it inflated, or in the CO2 case this is the low dKH scenario where a small amount of CO2 addition causes a large pH drop. If the castle has a lot of holes we need to add a lot air to keep it inflated, and in the CO2 case this is the high dKH scenario.
But, rather than estimating how many holes we have, we can have a hole of standard size that always lets out the same amount of air, for CO2 this is the "4dKH solution in the drop checker scenario", where we add a stable amount of air ("CO2") to keep the castle inflated ("CO2 at 25ppm").
In all these cases if we turn the compressed air (CO2) off, the castle deflates and the HCO3- ~ H2CO3 equilibrium returns to its atmospheric level.
Personally for me the air compressor is always going to be turned off, as I can find quite enough ways to accidentally kill my fish without deliberately adding another one.
cheers Darrel