Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Total alkalinity is how well the water resists changes in pH. This buffering can come from many different salts and other materials. The major thing in fresh water tanks that does this is bicarbonate.
Carbonate hardness is a measure of carbonates and bicarbonates.
It is my understanding that most hobby level KH tests are actually measuring how well the water resists a change in pH. (Alkalinity).
Since the most common buffer is the carbonate/bicarbonate/CO2 reaction, it is reasonable to call these tests KH tests.
I would do this:
Make up whatever amount of water you want to use for a water change.
Add about 1/2 of the minerals etc. you think are right.
Circulate the water for at least half an hour, but longer is just fine, to make sure they are all dissolved.
Test. The results should be almost exactly half the values you are aiming for.
If not, then do some math and add the remaining minerals. Circulate, test.
If the end result is still too soft, allow the water to circulate longer, perhaps overnight, and test again. Then add more minerals if needed.
If the end result is too hard, then add some water to dilute it.
No matter how carefully you do the math, it will always come down to actually making the recipe and testing it.
Once you have figured it out you will not have to wait until all the minerals are dissolved before adding the new water to the tank. I just set up the new water first, and run a small pump on it while I am doing the water change, cleaning the filter and so on. By the time I am ready to use the new water to refill the tank the minerals are well enough dissolved.