MYWAY
the 3hr comment....
My roomate is an Aquaculturist and we measured the PH absorbtion time. If you have a kiddie pool, then yes, obviously more water volume would increase the amount of PH that can be "absorbed"
Just as a measure for all though.....40lbs of rock, in a 50gallon rubbermaid. Had a 3 hour threshold for the first week! That is just to let you all know, that once a day water changes are not the most efficient way to reduce ph.
...and, on the past....I have never had a rock break due to pasta swells, and depending on your tank inhabitents...the past tunnels are very cool. but, also, try halfway inflating a rubber glove and making a rock mold around it...you get an AWESOME cave doing that.