I have soft slightly acidic tap water and I always have to make a cocktail when it comes down to water change time...
Liquid Calcium, Baking Soda, and Prime (I also add a little iodide for my inverts to help with moulting... some say unnecessary- I say $6 for a bottle that will last me two years is worth the peace of mind considering I have so little TDS in my tap water).
For me- to get to 7.5 and a GH/KH of 8 each- I need to add a little more than 1/2 tsp of baking soda per 5 gallons- and 2.5ml of liquid calcium per 5 gallons.
I'll throw a tums in now and again but I don't think it does much to change the entire water chemistry... I don't want my tank water too much different to the water I add during water changes so as not to shock my fish/inverts- thus no buffered substrate for me.
They need calcium in their diet to build shells- and calcium in the water (along with other trace elements- hence TDS) to help build a nice shell. pH DOES matter though.
Try sticking a piece of chalk in vinegar... (chalk is primarily calcium carbonate- also a primary ingredient in shells... vinegar an acid)... I suspect you'll see the chalk slowly disappear before your eyes.
A low pH will dissolve Calcium Carbonate- although the more acidic the quicker it will occur.
My fish primarily prefer lower pHs and TDS... my tap water would be perfect for them... I meddle with my water for the inverts because an incorrect water chemistry is more harmfull for them than the fish.
