Removing calcium and magnesium?

[catnip]

Richard, a question on CA (I think) percipitate which I hope you can help me with:

I'm stuck with using a pool service since we are absent from our pool for three weeks out of every four. I've been experimenting with the BBB method during the week I'm here, but the pool service uses Tri-Chlor pucks in an in-line feeder and shocks with Cal-Hypo, sometimes at high levels because we have had recent pool flooding and black/mustard algae problems. We've had to do a lot of vacuuming to waste to get rid of the dirt due to flooding...also used some floculant to settle out the dirt. The pool has been flood/dirt free for a month, but the algae is persistent on the shaded side of the pool...not terrible but clearly there.

After the most recent shock with 4 lbs. of Cal-Hypo, the pool went from very clear to moderately cloudy blue for around 12 hours and then fairly clear after 24 hours and eventually very clear. However, there was a residue of what looked white powder scattered around the bottom. The Polaris sweep moved it around but it settled fairly quickly, kind of like very fine snow flakes. Does this sound like calcium carbonate percipitate? Or could it be dead algae (though the quantity of white power seemed much greater than the relatively small amount of visible algae we brushed off)? I vacuumed the white residue to waste and otherwise the pool looks great now.

Here are the numbers:
Volume - 18,000 gal.
FC - ~20 drifting down to 3-6 after shocking with Cal-Hypo and using bleach to maintain.
CC - can't measure but latest pool store LaMotte test showed 0
pH - 7.8 reduced to 7.6 with 14 oz. of muriatic acid
TA - 75
CH - 375
CYA - 30

Thanks for your thoughts on this.