@crsublette ... Hi Charles - I just came to my computer and can't keep up with all the info you wrote. You covered many topics. I basically have a dedicated Koi pond with no problems whatsoever. As to the above, I run a 8.4 pH and a quite high kH, 175ppm and sometimes higher. Never concerned about crashes, let alone swings. You've been busy ... I have to read through all your comments .... Thx
Well, I should clarify a bit... You can still have pH swings as a result of carbon dioxide concentration with a KH of 175ppm, just tougher to happen at higher alkalinity and near impossible to happen if good water circulation is involved....
....as I mentioned... my well water's carbonate KH alkalinity is at 240ppm yet the pH still swung from 7.5 pH to 8.6 pH... This is due to carbon dioxide concentration... as organic decomposition continues underground, the resultant carbon dioxide penetrates the water, no way for the carbon dioxide to escape the water while underground, and thus increases carbon dioxide concentration of the water, lowering the water's pH, even though the water is surrounded by limestone..
...this is due to how carbonic acid (that is dissolved carbon dioxide in water) bypasses alkalinity and thus carbon dioxide is not buffered in the "traditional buffering" sense by alkalinity such as when any other acid is introduced... as below presents, you can see that carbon dioxide is actually the denominator of the pH equation.
pH equation is ...
pH = pKa + log([HCO3-]/[H2CO3]) ...
HCO3- is the bicarbonate anion concentration, which is the measure of carbonate alkalinity shown on our KH test.
H2CO3 (carbonic acid) is the dissolved carbon dioxide in water.
BUT... since alkalinity is the numerator of carbon dioxide, this allows a different relationship to happen in context of pH... carbon dioxide also has a strong electromagnetic bond to water so water circulation is used to help release the carbon dioxide... however, since carbon dioxide is lighter than water, if the water depth is shallow enough, then the carbon dioxide will eventually "float" to the atmospheric surface where it will escape... water circulation just helps to speed up the release of carbon dioxide... In the deep deep oceans, there are actually liquid carbon dioxide deposits... this is due to the lack of water movement and allows carbon dioxide to build up faster than can escape the water... Various fish species have different toxic carbon dioxide tolerances (some food fish is near 20ppm, whereas koi I think is around 5~10ppm). Fish know where these layers are in the ocean and so they stay in the water layers is most suitable to them.
Alkalinity, pH, and water circulation determines the amount of free carbon dioxide in the water... Free carbon dioxide is what is toxic to fish... Free carbon dioxide is a function of pH and carbonite KH alkalinity. Higher the free carbon dioxide, the more important water circulation becomes. You would have to have a tremendous amount of carbon dioxide, with little water circulation, to have noticeable amount of free toxic carbon dioxide in a higher kH, high pH water. Planted aquariums actually attempt to keep a high carbonate KH alkalinity while maintaining a low pH. To do this, requires minimal water circulation and carbon dioxide injection into the water.
So... You can still have pH swings as a result of carbon dioxide concentration with a KH of 175ppm, just tougher to happen at higher alkalinity and near impossible to have this type of "soft" (or diurnal) pH swing to happen if good water circulation is involved....
The carbonate alkalinity buffer system requires water circulation to be involved for this buffer to prevent carbon dioxide from impacting pH. Surface agitation is not necessary, except does make the process go much faster.
This is one reason why water circulation is quite important.
