Hardness Ingredients

Calcium carbonate is a major component in animal skeletons. You can add CaCO3 to your aquarium with crushed coral gravel, coral sand, or coral skeletons, but they may look out of place in a planted aquarium. It is not necessary to add a lot either.

Calcium carbonate also occurs in dolomite, which is pretty common, at least around here.

Graeme

I'm quite sure calcium carbonate has a low solubility (>1mol/L), so assuming this, there is no quick way to dissolve it in water.

Carbonates of Group 1 elements (Sodium, Potassium, etc) have a higher solubility. I'm not sure where you can get K2CO3 potassium carbonate), but you could try NaHCO3 (sodium bicarbonate). I have read that plants prefer low quantities of sodium, so the coral may be your best option.

Please let me know if I have made any mistakes,

Graeme

What's the rush on the dissolving of the crushed coral?
I lasts for months and does a great job of raising kH.
Put a bag of it into your filter and you'll start to see your kH rise in about a week or so. It's not fast acting but it works well.

Len

I dissolve about 1/8tsp of calcium carbonate in a jar of water and add that to my tank after each water-change. It turns the tank cloudy for about an hour or two, but dissolves completely.

Leaving crushed coral in your filter will slowly, but continually raise your GH/KH so after a water change your water won't be as hard as after a week or so. If you are adding a constant source of CO2 (like pressurized) this will cause your PH to decrease as your hardness increases. For this reason I add calcium carbonate instead of crushed coral in the filter... because it keeps my PH more stable.

Just my experience.