troubleshooting alkalinity

[kwirky]

I think I have it solved! Finally!

So i've been using Oceanpure salt, which is a great price, and it's loaded with everything. Mixes nice and fast. Only thing is I've got a strange hardness thing going on now.

Here are my water parameters:
temperature 80.2-4
specific gravity 1.025
120ppm CaCO3 (calcium carbonate) (6.72 dkH, 2.4 meq/L)
440ppm calcium.
whopping 1650ppm of magnesium.

So it's high in everything, INCLUDING magnesium. I'm suspecting my batch, since googling it, most everyone else gets 1350 ppm magnesium.
update: a bad batch has been ruled out

Here are some good readings I've found, which I think I should share.

reading on relationship between calcium and calcium carbonate:
http://www.advancedaquarist.com/issues/nov2002/chem.htm

reading on magnesium:
http://www.advancedaquarist.com/issues/oct2003/chem.htm

CaCO3 to meq/L to dkH converter
http://www.saltyzoo.com/SaltyCalcs/AlkConv.php

My pH fluctuates in VERY strange ways. I initially figured it to be unpredictable, but now that I've learned of magnesium's interference with the precipitation of CaCO3, it's very predictable.

saltwater with double the natural concentration of magnesium doubles the time it takes for CaCO3 to precipitate. Magnesium attaches to the newly forming CaCO3 crystals, inhibiting it's crystaline growth. So when dosing kalkwasser, the calcium is added right away, but the CaCO3 isn't. So that explains why my pH wouldn't raise until about 20 hours after the dosing of the kalkwasser. I was noticing my pH RAISING at night, rather than dropping.
update: I got this mixed up. It slows the precipitation of CaCO3. Meaning it slows the drop in your alkalinity. Am I right?



So the magnesium was preventing the calcium carbonate to form at the same pace at the calcium, I predict that since calcium had 20 hours to "have it's way" with the imballanced CaCO3, causing the alkalinity to drop further.

I've just been having wierd stuff happening.

Water changes are out of the question to correct the problem, because I tested a new batch of saltwater with the mix, and it tested high in magnesium as well. I don't want to blow money on another bucket of salt, since that high of a magnesium concentration would require a very wasteful water change. I'd rather try to fix it.

So I'm going to try adding baking soda. the formula is:
for 50 gallons of water
1 teaspoon of baking soda
raises alkalinity by 0.4 meq/L (~1dkH)

my target is 3.0 meq/L of alkalinity, so I'll be adding 6 tsp's of baking soda through a slow 24 hour drip.

I'll post my results.