Aragonite is much more water soluable than calcite (shells, crushed coral, dolomite, etc.)
I have several african cichlid aquariums with all aragonite substrate, I love it. Looks nice and helps keep the water very hard and a ph around 8.3 or so.
I don't think there is a sound answer to the question, "how much aragonite does it take to achieve a ph of (insert your desired ph) and a harness value of (insert your desired value). Way to many variables to have a good answer.
Some info that would help is,
How many gallon aquarium do you have?
What was the ph and harness values before you added the aragonite?
What is the ph and hardness value of your tap water?
What is your desired Ph and hardness values?
Even with this info, I don't think you can get a solid answer but it would be helpfull in getting a better answer. Reply with these answers and we'll see what we can figure out.
Joel