What would happen if I put oyster shell in my water?

[LinCa]

Here's my thinking:
My pH is currently 6.9 (any water I use to top the pool up is 6.8). So I need to increase pH.
My Total Alkalinity is currently 34 with a goal of around 100.
I have a vinyl pool so calcium hardness isn't critical.
My pool is really small, so if I do something terrible the worst I have to do is drain it and start again.

Also...
I keep hens and give them clean, crushed oyster shells as part of their diet.
Oyster shells are made of calcium carbonate.

And...
When sea water becomes even slightly acidic it is famously eating coral reefs and weakening the shells of sea creatures.

So...
What if I gave my pool clean, crushed oyster shells to eat?!
I could put some crushed shells in a mesh bag and suspend the bag in the pool water.

Why not?

 

[mknauss]

yes -- but why? Your vinyl pool could care less how much calcium is in your water. Why add it???

 

[LinCa]

Because my pool is a science experiment!

And because it might increase the pH and total alkalinity... Or wouldn't it?

 

[Donldson]

I'm lost. What is your reason for doing this? What do you intend to accomplish? What leads you to believe doing this will have any affect on your pH and TA?

If your pH and TA are really that low (big "IF" since you are using unreliable pool store testing) then you need to raise them with either borax and baking soda or with sodium carbonate.

 

[LinCa]

I'm lost. What is your reason for doing this? What do you intend to accomplish? What leads you to believe doing this will have any affect on your pH and TA? ...

I am wondering if calcium carbonate (CaCO3) can be used instead of/as well as sodium carbonate (Na2CO3) or sodium bicarbonate (NaHCO3) to achieve some of the same outcomes. I don't know the answer to that - which is why I am asking the question.

My only previous experience with managing the pH of water is when I used to keep tropical fish. The aim there was to raise or lower the pH very slowly to avoid harming the fish. For example fish-keepers who wanted to lower pH could add certain leaves to the water and allow them to slowly release tannic acids (oak leaves are good for that). If you wanted to raise the pH you could add oyster shells to the filter media. I never tried it because my water always sat at 6.8, which was ideal for the kinds of fish I kept.

So I am merely speculating about what kinds of water management techniques might be transferrable between fish keeping and swimming pool maintenance.

 

[mknauss]

Calcium carbonate is not soluble in water. Calcium chloride is. That is what is used to add calcium to water.

Be careful using borax or sodium carbonate. They move pH pretty quickly. Sodium bicarbonate, aka baking soda, moves TA, but pH not so much.

 

[LinCa]

Thanks, mknauss, that is very helpful info. In fact I have been adding sodium bicarbonate to the water to increase the TA, and can't even find borax for sale in my town. It used to be sold at every grocery store, but apparently not any more. I am planning to increase the TA as a buffer before specifically altering the pH.

 

[LinCa]

...
If your pH and TA are really that low (big "IF" since you are using unreliable pool store testing) then you need to raise them with either borax and baking soda or with sodium carbonate.

I am pretty confident that's an accurate pH reading. I have been measuring the pH in my water for many years and it has never varied from 6.8. The current reading of 6.9 is only because of the addition of baking soda and plenty of splashing!

 

[JamesW]

I would forget the oyster shells. Just use some baking soda to raise the TA in stages until the pH is right.

Typically, if you increase the TA, the pH will raise a little bit right away and more overnight.

Don’t target any specific TA; let the pH stability determine the TA. You shouldn’t need to add anything to raise pH. Go to 60 TA and check pH the next day.

Every TA will create a different pH stability point. The higher you want the pH, the higher you have to keep the TA.

Baking soda will raise pH any time the pH is below 8.3. The pH rise depends on how close the starting pH is to 8.3. At 6.9, baking soda will increase the pH a few tenths of a point in 24 hours depending on how much you add.

As the pH rises towards 8.3, the effect of adding baking soda decreases exponentially.

What source of chlorine will you use?

 

[LinCa]

Thanks JamesW. That’s a helpful explanation. The Pool Math calculator recommends 600g of sodium bicarbonate to increase the TA. This evening I have confirmed the TA and pH readings using my newly purchased test kit, and added 300g of baking soda. Tomorrow I will have a nice splash around before testing again.

I am very tempted just to take a bucket of water and put some oyster shells in it, and test the TA & pH over a couple of days to see what happens. Of course not all the parameters will be identical to my pool, but I still think it could yield some interesting results.

 

[LinCa]

What source of chlorine will you use?

I am using Focus brand stabilised chlorine [yes, that is the correct spelling...]. The active ingredient is sodium dichloro isocyanurate. Crikey, that’s a mouthful.

 

[JamesW]

Dichlor has lots of CYA, so not a good choice as a single source of chlorine.

Salt systems are good, liquid chlorine is also good choice.

The only time that I would use dichlor or trichlor would be when you want to raise the Cya.

Dichlor and trichlor are both acidic and will lower pH. When using dichlor or trichlor, you typically need to compensate for the acidity by adding extra baking soda.

 

[AUSpool]

...

I am very tempted just to take a bucket of water and put some oyster shells in it, and test the TA & pH over a couple of days to see what happens. Of course not all the parameters will be identical to my pool, but I still think it could yield some interesting results.

The pH has to be at or below 6.5 for calcium carbonate, shell or coral rubble, to dissolve. Calcium carbonate is dissolved in calcium reactors in elaborate marine aquaria setups but the pH is maintained by CO2 injection. The next issue is volume, even in a small pool, and assuming you could liberate some carbonate, it would be insignificant even in a small pool.

Calcium reactors work, with a bit of effort and expensive equipment, in aquaria due to the small aquarium volume compared to reactor size and the ratio of calcium and carbonate ions liberated is almost identical to the demand.

 

[cfherrman]

The pH has to be at or below 6.5 for calcium carbonate, shell or coral rubble, to dissolve. Calcium carbonate is dissolved in calcium reactors in elaborate marine aquaria setups but the pH is maintained by CO2 injection. The next issue is volume, even in a small pool, and assuming you could liberate some carbonate, it would be insignificant even in a small pool.

Calcium reactors work, with a bit of effort and expensive equipment, in aquaria due to the small aquarium volume compared to reactor size and the ratio of calcium and carbonate ions liberated is almost identical to the demand.

Very cool info!

 

[LinCa]

That is indeed very cool info, AUSpool! Thanks for taking the time to explain that.