I had quite a bit of Texas limestone in my old reef. This supplemented my liverock. I never had pH problems and in fact that reef was well-balanced; it rarely had pH, alk, or calcium shortages even though I only intermittently dripped kalk.
Over time the limestone was covered with coralline...
How much sand is going in the tank? If it's 4 inches, then you effective depth is that much less.
I only have a 30" high tank, but in my experience it is fairly rare that I actually have to reach to the bottom; most of the time if I'm reaching into the tank it's to help a coral that's close to...
I use a Magnavore VI on my acrylic tank and it works well -- it won't remove coralline, but used regularly it will keep it and other algae at bay. I clean with it about two times a week to prevent build-up (it takes about 10 min. on an 8 foot tank).
About once a month I scrape off any...