One very important question asked earlier in this thread hasn't been answered, and it is crucial. What is the GH [general hardness] and KH [carbonate hardness, or Alkalinity] of your tap water? And the pH too for that matter?
As someone mentioned, these three are interconnected, and changing pH will be risky and may be dangerous [as you have found out] without knowing and managing the GH and KH. Rather than go into the details, have a read of my article on this relationship: http://www.tropicalfishkeeping.com/f...quarium-73276/
Once we have the numbers [and you can get the GH and KH from your municipal water people, probably on their website] we will be better able to advise how best to achieve what you want.
The African rift lakes have some of the hardest freshwater in the world. And the GH is more important than pH when it comes to fish,generally speaking. Fish like the cichlids in the rift lakes require "hard minerals" in the water, primarily calcium and magnesium, and their metabolism is designed to suit this state. At the opposite end of the scale, soft water fish come from waters with no such minerals, and they have a physiology designed for this lack of any minerals. The two are not interchangeable.
There are easy and safe ways to increase GH (and corresponding pH). Using the rift lake mineral salts is one, though expensive long term. A less expensive method is to use a calcareous substrate, similar to the one you mentioned previously. A sand composed of dolomite/limestone/aragonite/crushed coral [any of these, or a mix] is best. These will naturally raise the GH considerably, which is fine for these fish. We can discuss further when we have numbers for GH and KH and pH.
A final comment on the store's pH 7 for all their tanks. This works short-term, but not permanently for fish. A store will usually have fish with varying water needs, and today most stores use a circulating water system whereby the water runs continuously from tank to tank and back to the filter. Adjusting individual tanks is thus impossible, unless they are on separate systems. But the store hopes to sell their fish stock within a few weeks, and with some exceptions, this "middle of the road" approach to water parameters is not usually critical to the fish. Bong long-term in the home aquarium is a very different thing.