I'm sorry you're having some trouble with your corals. I agree with others above that 7.2 is not a low alkalinity, and I don't think this caused your coral death.
I believe that with coral issues you should look at three big things, in no particular order: lighting, flow, and water parameters. Can you give us some more information about your tank that I think will help R2R better help you, such as:
How old is this tank?
What do you have for lighting?
What do you have for flow?
Can you post a picture of your tank?
Do you know your other water parameters, particularly nitrate and phosphate?
Do you know what the salinity of your tank is?
What kind of corals do you have that are dying, and are any of your corals doing well?
Buying water from your local store is fine, but you should always check it with a refractometer to ensure it's at the right salinity. Don't just assume that the water is fine because it came from a reputable store.
As RHF mentioned above, you can use baking soda to raise alk. And I agree with others who suggested choosing a salt mix that has an alkalinity where you want the tank to run, and then you simply have to maintain alk.
Last, if you're keeping soft corals they are less affected by low levels of Alk, Calc, and Mag than their stoney coral cousins because soft corals do not have to build a calcium carbonate skeleton.
I hope this helps!