Hi
I asked the guy at the LFS about their planted tank, which is using RO water, and I noticed that they were injecting C02 at a very slow rate; something like 1 bubble every two seconds, whereas when I was using C02 previously, I was having to pump it in at something like 6-8 bubbles per second to achieve a 1 PH drop to get C02 levels right. My tank parameters are PH 8.4, KH 11, GH 13.
Could somebody explain the relationship between the the PH, KH and the amount of C02 to reach the desired C02 levels please ?
Also, If I were to mix my current tap water with the above parameters,50/50 with RO, what would my resulting parameters be ? am I right in saying that my KH & GH would half ? what about PH ?
Thanks
Paul
Hello,
The amount of CO2 needed to be injected in order to reach a certain amount of dissolved CO2 is exactly the same whether using pure RO, a mix of RO/tap, or pure tap.
CO2 does not care about the alkalinity of the water. Only three things determine the solubility or retention of CO2 in water:
Temperature, Pressure, Salinity.
Having said that, it's necessary to understand that in water having a high alkalinity versus RO water, the pH changes that result from injecting the same amount of CO2 will be different. This difference is caused by the higher alkalinity which resists a change in pH. In fact, this is by definition, what alkalinity is. If you add the same amount of acid each to a sample of RO water, RO/tap and to a sample of KH 11 water the same amount of acid will enter the water, obviously, but the ph drop in KH 11 water will be slight. The pH change in the RO/tap water will be moderate and the pH change in the RO water may register as very large.
In our case, the acid that we are adding is a weak acid called Carbonic acid, which is formed when CO2 comes into contact with water. It is unavoidable and is complicated, however, it's only necessary to understand the different
behavior of pH when acid comes into contact with the various types of of water.
Also, you cannot really compare your LFS system with your own. Their injection pressure may be high, for example, set by their regulator working pressure. The temperature of their water may be different. The tank may be more tightly covered and may retain more CO2. Their injection method and flow/distribution in the tanks may be different, so it's really difficult to compare tank to tank.
If you cut your tap with 50% RO, then KH and GH would halved, but it is unclear by how much the pH would change.
pH is a logarithmic calculation. KH and GH are linear (proportional) calculations.
In any case, it is unimportant what the pH of the RO/tap mixture is.
It only matters what the change in pH is when you add the CO2.
That is the information you look for when trying to determine whether you are adding enough CO2.
If your resulting KH is around 4-6, then you can aim for a 1 unit pH drop. If the KH is higher due to using a greater percentage of tap then the pH drop when adding the same amount of CO2 will be less.
Hope this helps.
Cheers,