Backyard Aquaponics
A place to discuss aquaponics

Which one to use to buffer PH if growing lettuce only ? I have calcium carbonate and potassium bicarbonate

What is the pH currently?

PH is 6.3

So you will likely want a product that both effectively raises and buffers pH, to get it back up into the high 6's and more stable.

Calcium carbonate, if it's in the form of regular garden lime, which is basically crushed limestone and looks much like beach sand, will raise the pH a little initially, but not a whole lot. It will provide a slow release of carbonates, so adds a lesser immediate buffering effect, but over a longer period.

Potassium bicarbonate will also both raise and buffer pH. By volume it will raise the pH much more, far quicker, and provide more immediate buffering capacity than the Calcium carbonate.

But be aware, if your are having to add pH raising and/or buffering products on a regular basis, it's best to use a combination of a number products, or at least alternate between them, so as not to mess up the K, Ca, Mg balance too much. So the use of Dolomite lime in addition to the afore mentioned products would be a good idea IMO.

So my question is , do I need to add both to avoid imbalance between Ca , K , Mg ?

If I use only potassium bicarbonate on a small dosage , does it affect Ca and Mg ?

Using small amounts of Pot' bicarb 2 or 3 times in a row probably won't be an issue, but if you keep using it exclusively, after a while you will likely see a Ca and/or Mg deficiency in your plants.

I would add a fair bit of Ca carbonate (if it's in the form that looks like beach sand) in small amounts over a few days, ie: a heaped tablespoon per 500L. Each addition will increase your pH slightly, but you are primarily doing it to add a good deal of slow release buffering capacity.

After a few days, with the buffering capacity increased, and only if required (the Ca carbonate may have been enough), I would then add small amounts of Ca hydroxide (Brickies/builders lime) to raise the pH more effectively. About one level teaspoon (use proper measuring spoons) per 500L every day until you get the pH up to around 6.8. Try not to raise the pH by more than 0.3 per day.

Once you've achieved a pH in the high 6's you want to maintain the buffering capacity of the water as well as the pH. To do this you'd test the pH regularly and as soon as you see a small drop you'd add small regular additions of both Dolomite lime and Pot' bicarb in a ratio (volume) of around 3:1, then occasionally a bit more Ca carbonate as well. This should also keep the K/Ca/Mg in a decent ratio to each other

Hydroxides very effectively raise pH, but add no buffering capacity, so if you were to use them exclusively the effect would be very short lived and you'd have to add ever increasing amounts more often. Any of the products containing a carbonate will also raise your pH, but to a far lesser degree than a hydroxide by volume. They are primarily used to add and/or maintain pH buffering capacity.