Hey Mike,
I used Seachem Alkaline Buffer. The reason why the water changes aren't helping is probably because your tap simply has no buffering capacity (very low kH) and that never changes unless you buffer it chemically. Replenishing with water changes may stave off the plummeting pH simpy for the fact you are constanty replacing the low pH water with higher pH from the tap. Problem is you would have to do water changes all the time. One of the most important aspects of water quality is it's ability to stabilize the pH. Low or High pH is not really a problem for fish, they will aclimate, but a constantly swinging one will eventually cause all kinds of trauma. Every since I raised my kH to about 8, my pH stays at a nice 7.2. The only time I add any more buffer is when I do a water change. As I go I am trying to follow this rule of thumb:
Ammonia must be 0, Nitrite must be 0, pH must be within 0 to .2 of last test
kH must be within 5-9. Keep these balanced and you should be ok.