Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
"Two teaspoons (about 4 grams) of calcium carbonate per 189.3 Litres (50 US G.) of water will increase both General Hardness (GH) and Carbonate Hardness (KH) by approximately 1 degree" (Wiki)
So for 10 gallons, this would be 1/5 of 2 teaspoons, or a scant half teaspoon.
You want to make a stock solution so that 5 ml will have 1/2 tsp of CaCO3? I do not think you can make it that strong. Try, and start with RO or DI, and you might get more of it to dissolve if you add something acidic to it. Maybe use peat tea. I think you will have to adjust your dosing.
If you want to dose 1/2 teaspoon of dry CaCO3 in a 10 gallon tank, put that much in a cup or two of water (RO or DI, or peat moss tea) and shake it like crazy until it is dissolved. It does not dissolve easily.
Find another source of carbonates that does dissolve fairly easily. For example:
1 teaspoon of baking soda added to a 29 gallon tank will raise the KH by about 2 German degrees of hardness, or 36 ppm.
so... 1/2 tsp in 29 gallons = 1 degree
so... 1/6 tsp in 10 gallons = 1 degree
Try adding 1/8 tsp and let that work overnight. It dissolves fast, but let it get well circulated before you test. I would add that much directly to the 29 gallon tank, dry, right into the flow from the filter.