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.

[aquarium]

I have the swimming pool working okay with the high CH = 430ppm, but I'd like to lower it for my aquarium. (I used the aeration method to get alkalinity down, and that's a big hit.) Our water is unusual because the calcium:magnesium ratio is closer to 2:1 rather than the normal 3:1 or 4:1. I think the high magnesium is causing plant growth issues.

I adjust the water in a large trash can. Given a 35 gallon at a time working volume, is there a way to lower both calcium and especially magnesium?

[chem geek]

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.

With your numbers, you aren't severely over-saturated with calcium carbonate in your pool even at a pH of 7.8 while at a pH of 7.5 you are perfectly fine. So I do not believe the precipitate is calcium carbonate. Yes, dumping Cal-Hypo can produce some solids just as dumping some Sodium Carbonate (Soda Ash) can do the same (I've seen the latter myself), but this is pretty obvious clumps and even these tend to dissolve over time.

You can fairly easily determine if the stuff is calcium carbonate by taking some of it (scooped up) and putting it into clean water (tap water is OK, so long as it isn't really high in CH and TA). If the stuff dissolves (give it some time), then it is probably calcium carbonate; if it doesn't, then it could be algae.

Richard

[chem geek]

I adjust the water in a large trash can. Given a 35 gallon at a time working volume, is there a way to lower both calcium and especially magnesium?

I don't know of any other methods other than what was described in the earlier posts in this thread. For magnesium, I think that the water softener approach is much better than the precipitation method I suggested since precipitation is OK at removing calcium and is not as good for removing magnesium.

Richard