See, Let me start from defining Weak acids

A> Weak AcidsA weak acid is something that does not break in water Thats is- doesn't dissociate very much. It thus only handful of ions and remain as molecules.

So the same happens with Carbonic acid- It dosen't break in water and forms a few ions and yeah, they remain like molecules.

Any doubts? Send it my way! :)
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@Varunishah Please Mark the answer as best it helped you. :)
Carbonic Acid. the acid formed in a solution of CO2 and water, is a weak acid which means that it only partially dissociates in water. This is, I think, a huge advantage under some circumstances. I dose two part to maintain alkalinity. there is a direct Linear relationship between the dose rate and the alkalinity in my tank. At  best, I can reach an unstable, temporary equilibrium. Because carbonic acid is weak, it has reserves of carbonic acid that will, when an excess of H+ has been consumed by the dissolution of aragonite, give off more H+. it is, essentially, a buffered reaction. This means a calcium reactor forms a stable equilibrium with the equilibrium point shifting slightly as variables change.Because carbonic acid is a weak acid,calcium reactors are inherently more stable than other methods of maintaining alkalinity and calcium.
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