With your pH that low, there is likely some other buffer keeping it there. Crushed coral works great when there is no other buffer present, but with a 2nd buffer you might get some surprises.
At a pH of 5, there must be a considerable amount of buffer present. Adding a little carbonate will not change things. However, as you add more & more carbonates, it will eventually overwhelm the first buffer, and the pH suddenly jumps to 8 (the equilibrium pH of carbonate ... ie the cc). Unfortunately, in a mixed buffer solution, altering the buffer concentrations does not change the pH linearly. There is a big jump in the pH as the buffer concentration ratio hits a critical point. You cannot avoid that by adding the carbonate slowly, and your fish might not like that big of a pH change!
You have a couple of options. First you need to know your true pH. Since the wide range test only read to 5, the real pH might be even lower. You can read lower values with a pH meter (maybe get your lfs
to do it), or you may get the number from that published by your water co. <My water co. post water test results on line.>
If the real pH isn't too drastically low, and it is stable, you might consider acclimatizing your fish to the low pH rather than try to change it.
The other alternative is to use R/O or distilled water, then add crushed coral to that water to give yourself adequate buffering capacity, and acclimatize your fish to that pH. Then stick with R/O+cc for all of your pwc
, etc. <BTW - after reading your water co's analysis & finding out what is in the water that cause it be have such a low pH, you might just want to use R/O for your fish & yourself!>
If you want to try altering pH with just the cc in your tank, I would suggest doing that in a bucket to make sure there is no wild jump in pH. <You might be able to get away with just adding cc to your water, if it has many different buffers. In that case you might get a series of small jumps, rather than a big jump like in the 2 buffer system.>