Hi
2 part was a crude cut down very basic off shoot of the Balling system. As written above 2 part and also every other type of 3 part creates ionic imbalances and also do not add the same level of elements found in natural sea water.
Also no you would NOT have to add calcium to this system. As written above the system incorporates all elements including Calcium.
If you have a smaller system you just adjust the dosing accordingly, you could run this system on a 1 gallon pico system if you really wanted to
