Help me understand Ca/Mg amount vs. ppm - The Planted Tank Forum
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post #1 of 39 (permalink) Old 05-15-2020, 09:40 PM Thread Starter
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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?

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post #2 of 39 (permalink) Old 05-16-2020, 12:10 AM
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Iím grappling myself - looking to better understand dosing salts.

Have you had a play with the Rotala Butterfly nutrient dosing calculator?


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post #3 of 39 (permalink) Old 05-16-2020, 02:12 AM Thread Starter
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I’m grappling myself - looking to better understand dosing salts.

Have you had a play with the Rotala Butterfly nutrient dosing calculator?


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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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post #4 of 39 (permalink) Old 05-16-2020, 03:08 PM
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Originally Posted by mossman77 View Post
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.
To help out a bit with your questions. Once you become familiar with Rotabutterfly you will probably wonder how you ever got by without it.
To begin, you mentioned re-minerilizing your RO water. Do you use something to contain the RO water before adding it to your aquarium? (I do, 55g plastic drum). If so, follow along;
In Rotalabutterfly the first thing you are asked to do is input your aquarium size (or size of your RO water container). Lets use 55 gallons.

Then answer if DIY or pre-mixed. (Generally I do DIY. But if you are using something that is pre-mixed like Equalibrium then you would answer pre-mixed)
After answering DIY, you are asked to select your compound.
Calcium (Ca) powder from the various suppliers is CaSo4.2H2O.

Magnesium (Mg) crystals from the various suppliers is MgSo4.7H2O.
Sodium Bicarbonate (Baking Soda) is NaHCO3 (I have used this in the past, but the plants don't / can't really use the NaH)(The CO3 is what raises your dKH)
Potassium Bicarbonate (Amazon) is K2CO3 (I am now using this as the plants can use the potassium)(The CO3 is what raises your dKH)

Each of the above can be selected (as a compound) in Rotalabutterfly individually. Lets try MgSO4.7H2O. Then select Dry Dosing as you will be adding the crystals to your RO container.
Now you are asked "I am Calculating For:" - Select "Dose to reach a target". I selected 10ppm. Aiming for a 2:1 ratio of Ca:Mg
Now hit the calculate button.
On the right side of the screen it will tell you how many grams or units of measure to add to your RO container / aquarium to reach your target.


As you can see, it also lists the Mg, S, and dGH amounts. I typically use a spreadsheet to calculate how the individual amounts add up. For example, you should have been given a dGH of 2.31. If I change my compound to CaSo4.2H2O and set my target to 20ppm I will get a dGH of 2.8. So, adding these 2 compounds to your RO water will increase your dGH by 5.11.

Hope the above helps.


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post #5 of 39 (permalink) Old 05-16-2020, 08:16 PM Thread Starter
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Originally Posted by mossman77 View Post
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.
To help out a bit with your questions. Once you become familiar with Rotabutterfly you will probably wonder how you ever got by without it.
To begin, you mentioned re-minerilizing your RO water. Do you use something to contain the RO water before adding it to your aquarium? (I do, 55g plastic drum). If so, follow along;
In Rotalabutterfly the first thing you are asked to do is input your aquarium size (or size of your RO water container). Lets use 55 gallons.

Then answer if DIY or pre-mixed. (Generally I do DIY. But if you are using something that is pre-mixed like Equalibrium then you would answer pre-mixed)
After answering DIY, you are asked to select your compound.
Calcium (Ca) powder from the various suppliers is CaSo4.2H2O.

Magnesium (Mg) crystals from the various suppliers is MgSo4.7H2O.
Sodium Bicarbonate (Baking Soda) is NaHCO3 (I have used this in the past, but the plants don't / can't really use the NaH)(The CO3 is what raises your dKH)
Potassium Bicarbonate (Amazon) is K2CO3 (I am now using this as the plants can use the potassium)(The CO3 is what raises your dKH)

Each of the above can be selected (as a compound) in Rotalabutterfly individually. Lets try MgSO4.7H2O. Then select Dry Dosing as you will be adding the crystals to your RO container.
Now you are asked "I am Calculating For:" - Select "Dose to reach a target". I selected 10ppm. Aiming for a 2:1 ratio of Ca:Mg
Now hit the calculate button.
On the right side of the screen it will tell you how many grams or units of measure to add to your RO container / aquarium to reach your target.


As you can see, it also lists the Mg, S, and dGH amounts. I typically use a spreadsheet to calculate how the individual amounts add up. For example, you should have been given a dGH of 2.31. If I change my compound to CaSo4.2H2O and set my target to 20ppm I will get a dGH of 2.8. So, adding these 2 compounds to your RO water will increase your dGH by 5.11.

Hope the above helps.
Thanks for the explanation. I was selecting EI dosing so I wasn't seeing the correct information. I'll give it another shot.

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.

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Last edited by Darkblade48; 05-18-2020 at 02:29 AM. Reason: Please use the edit function for back to back posts to keep threads cleaner
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post #6 of 39 (permalink) Old 05-16-2020, 09:05 PM
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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?)


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post #7 of 39 (permalink) Old 05-16-2020, 09:46 PM
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Just as a bit of warning and not to try to make it seem more difficult, do go about this in a somewhat slow, methodical way, to avoid stressing any fish, etc. more than needed.
Different folks do react differently to changes in the water, so slow changes are much better to allow them to adapt to what the new water gives them. Radical changes in GH/KH can really stress some, so better to ease them into it slowly. And, as always, watch them to see how they are doing---just in case. Not a bad idea to have some of the old water held back in case you see too much going on and want to step back a bit.
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post #8 of 39 (permalink) Old 05-17-2020, 01:16 AM Thread Starter
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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?)
Isn't 1 degree equivalent to 17.8ppm? If 1/2 tsp of CaS04 provides 20ppm, thats for one liter of water isn't it?

I bought the CaS04 and MgS04 from NilocG.

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Just as a bit of warning and not to try to make it seem more difficult, do go about this in a somewhat slow, methodical way, to avoid stressing any fish, etc. more than needed.
Different folks do react differently to changes in the water, so slow changes are much better to allow them to adapt to what the new water gives them. Radical changes in GH/KH can really stress some, so better to ease them into it slowly. And, as always, watch them to see how they are doing---just in case. Not a bad idea to have some of the old water held back in case you see too much going on and want to step back a bit.
The tank is currently at a kH of 4 and gH of 8, so I'm not changing the parameters.

My apologies...the bags say 10 ppm per 10 gallons for the stated amounts. Makes more sense now.

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Last edited by Darkblade48; 05-18-2020 at 02:29 AM. Reason: Please use the edit function for back to back posts to keep threads cleaner
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post #9 of 39 (permalink) Old 05-21-2020, 05:01 AM
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I just use Remineraliz-P and baking soda does half baking soda to Remineraliz-P.
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post #10 of 39 (permalink) Old 07-12-2020, 02:05 AM Thread Starter
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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.

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post #11 of 39 (permalink) Old 07-13-2020, 01:10 AM Thread Starter
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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?

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post #12 of 39 (permalink) Old 07-13-2020, 01:48 AM
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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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post #13 of 39 (permalink) Old 07-13-2020, 02:12 AM Thread Starter
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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.
Thanks for the explanation! I needed a good example to work from. I'll do some calculations tomorrow and see exactly where I stand.

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post #14 of 39 (permalink) Old 07-13-2020, 03:11 AM Thread Starter
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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.
I just worked through the numbers and got the same thing, so that's encouraging. I've been setting my gH to around 7 (not sure if this is ideal for plant growth) and my RO is slightly mineralized (by a filter in my system--I should bypass it come to think of it, so I am uncertain exactly how much Ca and Mg I'm starting with. I'm dosing more or less 35 ppm of CaSO4 and 10 ppm of MgSO4. So I should dose 40 ppm of K per week correct? Three of my current macro doses provides 19.8 ppm of K per week, so I'm at half of what I should be?

When determining my Ca:Mg ratio, I use the ppm numbers not the resultant dgH, correct?

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post #15 of 39 (permalink) Old 07-13-2020, 03:56 AM
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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.
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