those buffers don't work like people think they do, waste of money most of the time.
It is better to just let the fish adapt or if your levels are on the extreme side, just buy fish that thrive in those conditions.
It is important, perhaps more so than PH, to know your other water parameters ammonia, nitrates, and nitrites.
I would suggest testing those levels and doing plenty of water changes instead of attempting to buffer the ph, because more likely it is water quality and tank size that are your problem and not ph.
Melfix should help the tail fin, pristine water should help the "mold" (fungus), a higher temp should help the ick. But, most importantly..water changes, water changes, water changes.