CaCO3 Solubility in Aquarium

[Fishworks]

Hey Everyone,

I just setup an aquarium with plants, and I am planning to add fish but I have some concerns. I want to have stable water parameters to minimize stress on the fishes.

The potential issue here is that I used a stone called Seiryu stone. Seiryu stone contains Calcium Carbonate which can alter PH, KH and GH if the PH is below 7.
From my understanding, if the aquarium water PH is 7 and above, then the water is neutral or basic and therefore will not dissolve the CaCO3.
Vice versa for when aquarium PH is below 7, the water will be acidic and thus dissolve the CaCO3.

My water from the tap is 8.1PH, after CO2 gas injection for the plants, it drops to 7.1 PH.
I should not expect the CaCO3 from the Seiryu stone to dissolve right?
Is my understanding here correct?

 

[Borek]

It is not a dissolves/doesn't dissolve (AKA yes/no) situation - CaCO₃ dissolves always, no matter what pH is. The lower the pH though, the easier the dissolution and the higher the concentration of Ca²⁺ in the saturated solution.

 

[Fishworks]

It is not a dissolves/doesn't dissolve (AKA yes/no) situation - CaCO₃ dissolves always, no matter what pH is. The lower the pH though, the easier the dissolution and the higher the concentration of Ca²⁺ in the saturated solution.

Alright, I think youre saying there is more dissolution of CaCO3 at lower pH. The carbonates will then increase kH while the Calcium contributes to an increased gH. I did some reading.. although I am still not confident in my knowledge.

Well would you say the rate of change at 7.1pH is insignificant? If its such a small rate of change, the fish should be fine right?
or I can only find out after I put the fishes in? give them time in the tank and let them tell me if they are comfortable or not?

 

[BillTre]

Generally speaking,
Adding CO₂ to an aquarium's water will lower its pH.
It can however, be "blown off" by equilibration (by aeration) with normal atmosphere relatively easily. So, its a temporary change.

Dissolving of rocks in the water will drive he pH toward the pKa of the major minerals involved.
A more crushed up rock (more surface area for dissolution to happen at), with more water flow over it (greater local concentration differences), will have a more rapid effect on the pH.
A lot of carbonates, like aragonite Ca/MgCO₃, have a pKa of about 8.3, which is also ~ sea water pH. They will dissolve at an increasingly slower rate, as they approach that value from below.
By taking advantage of this property, you can use thee materials to buffer pH changes.

Details will depend upon your tank care and maintenance. These are variables for you to sort out.
The ongoing biological processes of your tank will produce metabolically derived acids in the water, at some rate.
Water exchange rates will remove some water chemicals and replace them with others, at some rate.

 

[Borek]

A lot of carbonates, like aragonite Ca/MgCO₃, have a pKa of about 8.3

Can you elaborate? How do you define pKa in this context? "pKa of a mineral" doesn't sound like a standard Ka/pKa application in chemistry, but I know other trades often use terms in their own way.

 

[BillTre]

Can you elaborate? How do you define pKa in this context? "pKa of a mineral" doesn't sound like a standard Ka/pKa application in chemistry, but I know other trades often use terms in their own way.

OK.
The way I think about it is these "minerals" are for aquarium use are often somewhat variable mixtures of specific chemicals (like aragonite), with more or less proportions of Ca vs. Mg.
So kind of an average value of a chemical mix.
I am also guessing that they were formed in sea water (like aragonite) and have an "average" pKa based on deposition in those conditions (~ pH 8.3).

 

[chemisttree]

I guess you could just start with a few goldfish and a plecostomus. Hard to kill.