Frequency of pH Adjustment

[aquarium]

At PoolDocs sticky on adjusting alkalinity he says:

Also, do NOT try to lower your pH all at once; for all sorts of reasons, it's important to do it gradually!

What would those "all sorts of reasons" be?

I add about two cups of muriatic acid to a 15,000 gallon IG plaster pool every day to keep pH in the mid 7's, but we have very high alkalinity and hardness here. That bumps it down about .2 on the pH scale each time, but it's always creeping up by about the same amount each day. I also add about 4 cups of bleach per day, so that is also pushing the pH higher.

[waterbear]

I think what Ben means is that you don't want to dump a lot of acid in all at once to lower the pH by a lot. It is too easy to overshoot your target! Also it really takes about 24 hours for the pH to stabilze after ajusting it. It is much safe to make a small change and keep doing this over a few days until you get it where you want it and then add just enough acid to keep it there for maintenance.