TLDR: This is not asking what is the optimal ratio. This is a chemistry math problem for how to achieve a 4:1 ratio, regardless of whether or not that's the right or important "goal". Upshot is, historically this board and another have had a lot of math bandied about that show two results, one that Epsom Salt and Calcium Chloride (water content included or not) should be in about a 1:1 weight ratio, and one that shows a 1:2 ratio. Which is right??
There are two very different schools of thought in the mid2000s on this board, and I can't tell who's math is wrong.
In seeking a target of 67 dGH in 43 gallons of 0 dGH water using RotalaButterfly, it tells me that if I use 15 grams of CaCl2 (I'll get into Anhydrous vs Dihydrated in a moment) and 15 grams of Epsom Salt (MgSO4 x 7 H2O:
It will be:
(w/ Anhydrous CaCl2) 33 ppm Ca, 4.6 dGH and 8.5 ppm Mg 2 dGH (total 6.6 dGH)
(w/ didhydrate) 25 ppm Ca, 3.6 dGH / 8.5 ppm Mg 2 dGH (Total 5.6 dGH)
The ratio is 3:1 for dihydrate and 4:1 for the anhydrous. So no matter WHAT level of moisture is in my unknown bag of Calcium Chloride, adding 15g each Epsom Salt and CaCl2 PER THIS MATH would be between 34:1 Ca/Mg ratio and 5.66.6 dGH.
PERFECT water! Here is the problem!!
That math matches up VERY close with this (he calls for 6:7 ratio, I made it easy by saying 15g each):
https://www.plantedtank.net/forums/1...msulfate.html
ThomE, 2005:
Quote:
about one gram of mixed calcium chloride and magnesium sulfate (mixed 6:7 by weight) will raise 10 gallons of water by 1 degree general hardness (1 dGH) and provide an ideal calcium to magnesium ratio (3:1)
by Kevin Zippel, Ph.D

and then again, here, some math on this site. This matches up exactly with a 1:1 ratio.
https://www.aquaticplantcentral.com/...cl2mgso4.html Quote:
Short answer: Niko's calc + Fertilator
Long answer:
Starting with Mg portion of target:
Ca ppm = 4 x Mg ppm
(2.497 x Ca ppm) + (4.119 x Mg ppm) = GH ppm
9.988 Mg ppm + 4.119 Mg ppm = 17.9 ppm
Target Mg ppm = 1.27
Target Ca ppm = 5.08
MgSO4+7H20 dose (note: using 55gal, but you should substract 1520% for displacement if you are anal):
Target Mg ppm x 3.77L/gal x (mg MgSO4+7H2O4)/mg Mg x Tank gal = MgSO4 mg dose
1.27mg Mg/L x 3.77L/gal x (10.1mg MgSO4+7H2O4)/mg Mg x 55gal = 2660mg MgSO4 =~ 2.7g MgSO4
CaCl2 dose:
Target Ca ppm x 3.77L/gal x mg CaCl2/mg Ca x Tank gal = CaCl2 mg dose
5.08mg Ca/L x 3.77L/gal x 2.77mg CaCl2/mg Ca x 55gal = 2917mg CaCl2 = ~ 2.9g CaCl2

Later, in the same thread I linked FIRST above, we have our first detractor. This gentlemen now claims a more than 2:1 Ratio of CaCl2 to MgSO4 is required for the 4:1 ratio.
Hypancistrus, 2005:
Quote:
That's WAY WAY too much magnesium. Most people either do a ratio of 3:1 calcium:magnesium or 4:1 calcium:magnesium.
The numbers for a 4:1 ratio are:
For 1 dGH per gallon  0.07939 grams CaCl2*2H2O, and 0.03327 grams MgSO4*7H2O (epsom salt).
So, to raise 50 gallons by 5 dGH, you want 19.85 grams of CaCl2*2H2O and 8.32 grams of MgSO4*7H2O.

And now, in the APC Link above, math that backs up this quote:
Quote:
I haven't totally figured out czado's methods but he is incorrect. A degree of German Hardness is equivalent to 10 ppm of calcium oxide (CaO) or 17.86 ppm of calcium carbonate (CaCO3). CaO contains 7.1469 ppm of calcium and 2.853 ppm of oxygen. CaCO3 contains 7.1469 ppm of calcium and 10.7145 ppm of carbonate (CO3). See my work below.
To raise gH by 1 dgH in 1 gallon of water using calcium chloride dihydrate and magnesium sulfate heptahydrate while maintainig a 4:1 ratio of calcium to magnesium cations:
 Add 0.07939 grams of CaCl2*2H2O
 Add 0.03328 grams of MgSO4*7H2O
For 55 gallons, this would be 4.37 grams CaCl2*2H2O and 1.83 grams of MgSO4*7H2O.
Use this as a starting point. There are other factors that will affect the actual gH reading  calcium and magnesium cations may not register 100% equally on your test, the quality of the compounds used (agricultural grade from Greg Watson vs. reagent ACS grade from a lab supplier), compounds in your specific water chemistry  so simply adjust the measurement by using the same percent of each. (Example, if it raises it a bit more than you wanted, try multiplying each measurement by .75 to dose at 75% strength next time.)
DO NOT DISSOLVE CaCl2 and MgSO4 IN THE SAME CONTAINER!  The calcium and sulfate ions will bind and calcium sulfate will precipitate out. Calcium Sulfate is "insoluble"  it will dissolve but verrryyy sloowwllly.

Calculations  Assumes an EDTA chelation gH test in which calcium and magnesium cations register equally
 Calcium = 40.078 g/Mole
 CaCl2*2H2O Dihydrous Calcium Chloride = 147.01396 g/Mole
 Magnesium = 24.305 g/Mole
 MgSO4*7H2O Heptahydrate Magnesium Sulfate = 246.47556 g/Mole
1 dGH (Degrees of German Hardness) for calcium = 7.14691 ppm or mg/Liter / 1000
= .00714691 g/Liter / 40.078 g/Mole = .00017832 Moles
1 dGH (Degrees of German Hardness) for magnesium:
.00017832 * 24.305 = .0043342 g/Liter * 1000
= 4.33419 mg/Liter or ppm
Increasing 1 dGH of Calcium (7.14691 ppm) in 1 liter of water using Dihydrous Calcium Chloride:
7.14691 / (40.078 / 147.01396) = 26.21626 mg/Liter
Increasing 1 dGH of Magnesium (4.33419 ppm) in 1 liter of water using Heptahydrate Magnesium Sulfate (Epsom Salt):
4.33419 / (24.305 / 246.47556) = 43.95274 mg/Liter
Converted to Gallons:
Increasing 1 dGH of Calcium in 1 gallon of water using Dihydrous Calcium Chloride:
26.21626 * 3.785412 = 99.23934 mg/Gallon
Increasing 1 dGH of Magnesium in 1 gallon of water using Heptahydrate Magnesium Sulfate (Epsom Salt):
43.95274 * 3.785412 = 166.37924 mg/Gallon
Converted to a 4 : 1 ratio of calcium : magnesium
99.23934 mg/Gallon CaCl2 = 1 dGH
166.37924 mg/Gallon = 1 dGH
79.39147 mg/Gallon = .80 dGH
33.27585 mg/Gallon = .20 dGH

So here we have a lot of math, which is ALL from 2005 (though the chemistry hasn't changed! but the posters around here sure have!) where some are saying 1:1 and some are sayin a little bit over 2:1 of these two compounds is what is needed. RotalaButterfly apparently sides with the 1:1s.
What say y'all??