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Help me understand Ca/Mg amount vs. ppm

5K views 38 replies 8 participants last post by  mossman77 
#1 · (Edited)
I do my water changes with remineralized RO. I raise kH with sodium bicarb and gH with Ca and Mg. I'm confused as to how much of each to add, and how ppm is calculated. For instance, I just added a 2:1 ratio of Ca:Mg to 10 gallons of WC water. The Ca bag says 1/2 tsp is 10ppm and the Mg bag says 3/4 tsp is 10ppm. After adding 1/2 tsp of Ca and 3/8 tsp of Mg, the gH of my WC water increased 1 degree/17.9 ppm. 10 gallons equals 38 liters, so shouldn't it have taken 38 times that amount of salts to raise 10 gallons of water 1 degree?
 
#3 ·
I've glanced at it a couple times, but haven't spent much time trying to fully understand it. I'm confused by the nomenclature. For instance, what the heck is MgS04.7H20?

Until I figure it out, I'll just continue with the 2:1 Ca:Mg ratio and the NPK ratios listed on GLAs website.
 
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#6 ·
Okay, so I tried the MgS04.7H20 to obtain 80ppm in 10 gallons of water. It said I would need just over 2 tbsp. However, the MgS04 bag says 3/4 tsp raises gH by 20ppm.

Am I misunderstanding the bag? To me, this says 3/4 tsp of MgS04 weighs 20mg and thus gives you a concentration of 20mg per liter of water.
You are correct - just over 2 tbs would generate 80ppm and 18.47 dGH (which I would highly NOT recommend).
1/2 and 1/4 tsp of MgSO4 would give you 10ppm and 2.31 dGH. (a good starting point for Mg).
Generally speaking, Gh (or Kh) is seldom referred to in ppm. Typically the German degrees method is used. Water hardness converter is a good site to convert ppm to dGH. Regardless, 3/4 tsp of MgSO4.7H2O added to 10 gallons of water will not give you 20ppm GH. 38.5 ppm GH, yes.


When you say "the MgSO4 bag" - who's bag are you referring to? (manufacturer?)
 
#10 ·
I'll have to play around with the calculator some more to gain a better understanding. In the meantime, I've been cutting my tap water with 50% RO, which gives me a kH of 3 and gH of 3, then increasing my gH to 7 or 8 using a 2:1 ratio of Ca to Mg.

An Aquasabi article I read said to aim for a 2:1:0.5 ratio of Ca:Mg:K. I am EI dosing my 20 gallon tank at the 10-20 gallon dosage (1/8 tsp KN03, 1/32 tsp KH2P04, and 1/32 tsp K2SO4). How do I calculate the total potassium as it relates to the Ca and Mg I am adding to my water change water weekly? My gH is 7.
 
#11 ·
The Rotala Butterfly calculator is giving me 15 ppm of Ca, 5 ppm of Mg, and 7.5 ppm of K. The Ca:Mg ratio is 3:1, which is good, but I expected the K to be less (such as 3.75 ppm). Can anyone back up the supposed ideal Ca:Mg:K ratio stated on the Aquasabi website?
 
#12 ·
The Ca:Mg ratio is fine, but some prefer 3:1 or 4:1 (I'm in the 2:1 category). However, the K should be, at least, equal to Ca, e.g.; I run 30ppm K, 20ppm Ca and 10ppm Mg. Although my plants use Mg at a faster rate than K and Ca, they seem to do better when there is plenty of Ca and K in the water.

If I plug in 20 gallons of water into the Rotala calculator and use the "Result of my dose" drop-down in the "I am calculating for" section (e.g.; 1/8 tsp), I get:

1/8 tsp of KNO3 is adding 4.73ppm K
1/32 tsp of KH2PO4 is adding .66ppm K
1/32 tsp of K2SO4 is adding 1.19 ppm K

For a total of 6.58ppm K.

If it were me, I would add KNO3 at whatever NO3 target you want and KH2PO4 at your desired PO4 target. Then sum the resulting K contribution from those two doses. Subtract that summed K from your target K and dose that amount of K with K2SO4 (my K target would be 20ppm if Ca is 15ppm). For example, using the above doses of KNO3 and KH2PO4, you are adding 5.39ppm K. Subtract that from 20ppm K and you would add 14.61ppm K via K2SO4. So, you switch the "I am calculating for" section to "Dose to reach a target" and put the 14.61ppm K into it and the result will be that you add 1/4 tsp + 1/8 tsp of K2SO4.
 
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#13 ·
Thanks for the explanation! I needed a good example to work from. I'll do some calculations tomorrow and see exactly where I stand.
 
#15 ·
Your dGH is fine for plants, although many of us are lower. Your Ca and Mg dosing lines up with a dGH of about 7. If you actually dosed to reach your above-mentioned 15ppm Ca and 5ppm Mg, your dGH would be 3.25, which would be my preference and, remember, you have to adjust dosing to target when you do water changes, e.g.; if you do a 50% water change, then you add back 50% of the initial dose plus any lost to uptake. This requires testing for Ca and Mg to be more certain. If you are interested in this testing, let me know and I'll give you the detail of how to do it.

You may want to download this calculator: Zorfox's Planted Tank Calculator. In addition to doing the same type of calculations as RotalaButterfly, it also has a GH calculator, which would answer many of your questions.

Regarding how much K to dose. Yes, given the 35ppm Ca, I would dose to reach about 40ppm K, if you want to maintain that Ca level. Some members are ok matching the Ca level with K.

When building the ratios, you have to use ppm, since the dGH doesn't work the same way as the raw ppm numbers.
 
#16 ·
Great.. Thanks again. I feel like I'm finally getting a grasp on all these calculations. Regarding the Ca:Mg:K ratio, is this something I should tweak based on what plants I have and how they respond, or should I stick with the 1:1 Ca:K? Any comment(s) on why that Aquasabi article said 2:1:0.5 is an ideal ratio?
 
#25 · (Edited)
MgSO4.7H2O
Means 7 molecules of water are bound to the MgSO4.
Practical meaning is it weighs 7 water molecules more than MgSO4 non-hydrated.

You need to account for the water weight to determine the actual amount of Mg..

Maybe this was mentioned before..
120.366 g/mol MgSO4
246.47 g/mol MgSO4 . 7H2O

Hydrated form has 1/2 the magnesium as the non-hydrated form on a gram to gram basis approx..

Magnesium sulfate is a chemical compound, a salt with the formula MgSO
4, consisting of magnesium cations Mg2+
(20.19% by mass)
1g MgSO4 = .2019G Mg
.1008g Mg in a gram of MgSO4.7H2O (Epsom salt)

48.8% would be more exact than 50%

IF I remember my chemistry right.. Big if.


100.8 mg/gram of epsom salts
1 gram in 36gal (est of 40gal tank) 136.3L
100.8/136.3 = 0.740ppm..
To ADD 5ppm = 6.75 Grams.. Nobody should use spoons.. :)
Checked calcs here..
https://www.flowgrow.de/db/calculat...lizerEntityId]=7798&components[12][dose]=6.75

https://aquariumscience.org/index.php/4-6-salts-in-the-water/

I know all a we bit o/t but a bit bored today..
 
#31 ·
MgSO4.7H2O
Means 7 molecules of water are bound to the MgSO4.
Practical meaning is it weighs 7 water molecules more than MgSO4 non-hydrated.

You need to account for the water weight to determine the actual amount of Mg..

Maybe this was mentioned before..
120.366 g/mol MgSO4
246.47 g/mol MgSO4 . 7H2O

Hydrated form has 1/2 the magnesium as the non-hydrated form on a gram to gram basis approx..

Magnesium sulfate is a chemical compound, a salt with the formula MgSO
4, consisting of magnesium cations Mg2+
(20.19% by mass)
1g MgSO4 = .2019G Mg
.1008g Mg in a gram of MgSO4.7H2O (Epsom salt)

48.8% would be more exact than 50%

IF I remember my chemistry right.. Big if.


100.8 mg/gram of epsom salts
1 gram in 36gal (est of 40gal tank) 136.3L
100.8/136.3 = 0.740ppm..
To ADD 5ppm = 6.75 Grams.. Nobody should use spoons.. :)
Checked calcs here..
https://www.flowgrow.de/db/calculat...lizerEntityId]=7798&components[12][dose]=6.75

https://aquariumscience.org/index.php/4-6-salts-in-the-water/

I know all a we bit o/t but a bit bored today..
Boy, this is getting complicated. Isn’t the idea behind EI dosing to dose a set amount per week followed by a 50% water change, and the levels will eventually accumulate to where they need to be? And if EI supposedly doses in excess, then why is K so lacking?

If I dose the recommended NPK (1/8 tsp, 1/32 tsp, 1/32 tsp) three times per week, won’t my potassium level eventually reach what it needs to be? Do I really need to double my K dosing, and if so, why does the EI method use so little K? Is it primarily based on the Ca and Mg being dosed? Users should tweak the standard numbers for their particular Ca/Mg concentration?
 
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