there shouldn't be any swings, it should stay mainly acidic since I replace the almond leaves in my tank every 2 weeks or so. the water stays a dark brown color all year. not sure how fast old snail shells leach into the water but that will be the only other source of kh in the tank, and I use an air pump rather than co2 so that shouldnt effect the ph. and I can remove most of the shells if I notice the ph or kh start to rise
Dependent on the pH of the water, the air pump is likely to increase pH.
Consider this relationship between CO2 and carbonic acid
Notice how the arrows go both ways. So increasing CO2 increases carbonic acid, and increasing carbonic acid increases CO2.
As pH lowers, the water contains increased H+ ions, which bond with bicarbonate and carbonate to form carbonic acid. This carbonic acid will maintain an equilibrium with CO2 + H2O. More carbonic acid (lower pH), more CO2.
However, since water will want to maintain an equilibrium with the surrounding atmosphere (Henry's law
), any increase in CO2 molecules in the water will escape into the surrounding atmosphere. An air pump will increase surface agitation, increasing the rate at which the water and air reaches equilibrium. ie: It will drive off (any excess) CO2 (above equilibrium with the atmosphere) faster, and since there is less CO2 in the water (because the air pump is pushing the excess CO2 into the atmosphere), some of that carbonic acid will convert back to bicarbonate, increasing pH.
H+ ions (acid) from the almond leaves are a form of total alkalinty
. However it should be noted that H+ ions already in the water
do not increase total alkalinity. H+ ions already in the water are easily used in the process of converting carbonic acid to bicarbonate to carbonate, and vice-versa.
of H+ ions to the water will however buffer the water. The pH at which this occurs will be dependent on the amount of bicarbonate and carbonate in the water. With a source of bicarbonate and carbonate in the water, the H+ ions will react with these carbonates to form carbonic acid, which will maintain equilibrium as I described above.
However, if the bicarbonate/carbonate source depletes, the pH can drop rapidly to a very low value, since there's no more carbonate sources to maintain equilibrium. ie: The pH will buffer at a very low value.
Old snail shells are caclium carbonate (CaCO3). So they provide a carbonate source to the aquarium, and since they are a carbonate, and not a bicarbonate, they provide the strongest form of total alkalinity thanks to the carbonate ability to accept two H+ ions, compared to bicarbonates ability to only
accept one H+ ion. The rate at which the shells dissolve into the water is determined by how acidic the water is. Any H+ ions (acid) added to the water will bond with carbonate in the shells. More H+ ions added, more bonding, faster dissolution.
Hopefully that all made sense. If it didn't, or otherwise, my recommendation would be to start with a higher carbonate source in the water then might otherwise be needed. This will
provide protection against any errors, mistakes, or circumstances that were not considered. ie: If something goes wrong, you have a significantly better chance of fixing the problem before equilibrium of the chemical species is lost, resulting in the carbonate equilibrium shifting to the left, crashing pH.
The pH will stabilize at some value above your desired pH. With monitoring of the carbonate hardness (KH), you can then begin to remove some of the carbonate source from the water, through the removal of snail shells (or other carbonate source) from the water , or the reduction of (sodium/potassium) bicarbonate dosing. This reduction in the carbonate source will lower the pH. It should be noted that this is a balancing act between the amount of acid added. Adding more almond leaves will dissolve the carbonate sources faster.
You did imply these are the steps you would take, by removing shells if KH increases. A little chemistry theory doesn't hurt though..........right?