Sorry it takes me so long to get back sometimes, it can't be helped. Unfortunately, I have to ask more questions...
Do you have a quarantine tank available?
Can you do me a favor and take a cup of your tank water, as close in temp as you use to put into your tank during water changes, test the pH and then let it sit out overnight, untouched by anything fish related, and then test pH again after 24 hrs? Bring those readings here. I am trying to determine if there is a buffering problem in your tap water (unlikely but not impossible) or if the problem is in the tank specifically.
I agree with Byron on the crushed coral... all you would be doing is masking a problem that would likely worsen because it hasn't been resolved. Crushed coral, at this point, is not your answer. Also, please remember that I still suspect the problem was the Prime water conditioner and it is going to take time to remove it all from your tank water, even with daily water changes.
Each change dilutes it, but it is still in there and likely still at a somewhat high level. Seeing that your jump went from the 6.4 it started with to 6.8 now, that tells me you are making progress. Continue as you have been with the small daily water changes and no Prime. Continue tracking the pH shifts and reporting them here as things change in either direction. This is difficult because we are doing this via internet, I don't have the ability to see anything in person, test anything in person, etc. I am relying on you to keep us informed, and sometimes the best thing anyone can do is be patient and diligent.
I'm also very sorry for your frustration, but things like this happen. When you work with an aquatic situation in any form you have an ever changing environment. There was a time when people thought that fish keeping was as simple as dropping a fish into water and watch it swim. Over the years people have learned a lot, including that fish keeping, when done right, is a little more complicated than first thought.
Over those yrs many others have taken advantage of the still not widespread knowledge about this hobby and they create products to "help" us, though some of us are not actually helped by them. Buffering chemicals fall into that category. In all my 20+ yrs of working in this industry, keeping my own at home, etc, I have seen very few cases that actually required the use of buffering chemicals.... and too many cases where buffering chemicals/methods wiped out entire tanks of otherwise healthy fish, simply because the fish keeper was not patient enough to sort out the cause of their problems...
Quick fixes, in fish keeping, are never the solution. Hang in there, please. Don't give up. Once we figure this out you will be on your way to enjoying the fish because the "work" part will be greatly reduced, as will the stress of the problems you face at present. If nothing else, at least consider this a good crash course in water chemistry, something every fish keeper should have anyways. Everyone's tap water is different. Every tank is different. You are doing all you can right now, and should be proud of yourself that you care enough to sort this out and not impatient enough to allow the fish to suffer needlessly. There is an end... we will get there, and I promise to stick with you until this is resolved. The best is yet to come!
I will watch for those pH results from the cup of tap water and we will continue from there.