Aquarium Plants, Aquatic Plants, Planted Aquariums, and Aquarium Plant Fertilizer He carries all the dry chemicals useful to our hobby. The prices are very good and he is a good guy and a long standing supporter of the planted tank hobby. He carries several kinds of Ca sources, the easiest one to use is CaCl2+2H2O. CaCO3 is OK but does not dissolve easily (less than 1g per liter) CaCl2+2H2) dissolves much easier. The Cl should not hurt anything in the levels you would be introducing it.
Coincidentally, I was just figuring out a reference solution to calibrate your test kit using CaCl2+2H2O.
Step 1.
Dissolve 1.31g CaCl2+2H2O in 500ml distilled H2O
This yields a solution with 714.4mg/l Ca.
There are 7.144 mg/l Ca in 1 degree Gh
So, your solution has a value of 100 degrees
Step 2.
Dilute 10ml solution with 90ml distilled H2O.
This makes a 10% solution so its value is 10 degrees.
Step 3.
Test this dilute solution with your test kit.
If the reading is not 10 degrees, divide 10 by your reading and this is the factor your kit is off.
Example: you get 8 using the 10 reference solution.
10/8=1.25 so if you test your aquarium and get 6 degrees, you actually have 6*1.25= 7.5 degrees