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#### Fishworks

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Fellas,

I am now learning about how to dose the Secondary Macros Calcium and Magnesium.
I've read its 3:1 ratio between Ca:Mg. Also, im supposed to dose 1dGH worth after my weekly water change.
So then its dosing 15ppm for Calcium and 5ppm for Magnesium.

I am supposed to dose in CaCO3 equivalents right?

My calculations for Calcium are as follows:
Calcium dose 15ppm = 15mg/L while Tank is 400L.
15mg/l x 400L = 6000mg = 6g of Calcium. I want to dose 6g of Ca.
I'm using CaSO4.2H20 which is 22.5% Calcium.
Since I need to dose 6g of Calcium, then want to dose 26.67g of CaSO4.2H2O. Thats about 1.6 Tbsp.

However, if I were to dose in pure ions, I would have to divide 15ppm of Calcium by 2.5.
15ppm Ca in CaCO3 equivalents will become 6ppm pure ions of Calcium.
And then I do the above calculations again.

Thanks

#### ElleDee

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You should check out the nutrient calculators at Rotala Butterfly | Planted Aquarium Calculators & Information to help you figure out your dosing. I think you can get all your math questions answered there.

As for what you should be dosing, it depends. The 3:1 ratio is commonly cited, but it's not a hard rule. I'd say most people fall between 2:1 and 4:1. How much hardness you actually need to add is going to vary based on the water you're using and what your target is. I skew to a lower ratio in what I actually add because I have a little bit of calcium but basically no magnesium in my tap water.

#### Fishworks

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You should check out the nutrient calculators at Rotala Butterfly | Planted Aquarium Calculators & Information to help you figure out your dosing. I think you can get all your math questions answered there.

As for what you should be dosing, it depends. The 3:1 ratio is commonly cited, but it's not a hard rule. I'd say most people fall between 2:1 and 4:1. How much hardness you actually need to add is going to vary based on the water you're using and what your target is. I skew to a lower ratio in what I actually add because I have a little bit of calcium but basically no magnesium in my tap water.
I am asking if the 15ppm Calcium and 5ppm Magnesium is expressed as CaCO3 equivalents?

#### ElleDee

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Yeah, ok, I don't understand the utility of figuring things in CaCO3 equivalents in this application. Maybe someone else can be more use.

#### Fishworks

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Yeah, ok, I don't understand the utility of figuring things in CaCO3 equivalents in this application. Maybe someone else can be more use.
The rotala calculator has just left me confused.
Tom Barr said I should dose 1-2dGH or Ca 15ppm and Mg5ppm.
The result calculator Ca 15ppm is 2.1dGH (from CaSO4.2H2O) and Mg 5ppm is 1.15dGH (MgSO4.7H2O) for a total of 3.29dGH.

Why is the conversion so far?

Should I dose in 1dGH? or in Ca 15ppm and Mg 5ppm ?

#### Savetheplants

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7.144 mg/l Ca = 1 dGH
4.355 mg/l Mg = 1 dGH

I don't understand ion equivalent but this helped me. I hope it helps you. I admit I might not be following this well.

#### ElleDee

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I see what you're saying that 15 ppm Ca + 5 ppm Mg is not the same recommendation as "1 or 2 dGH", it's a bit more more. I don't know the context of these recommendations though. Perhaps he doesn't mean they the same, but that 1, 2, or 3.29 dGH are all plausible dosing levels. Like, keep your 3:1 ratio and aim for somewhere in there. Rotala butterfly isn't doing the math wrong. Maybe he misspoke a bit?

I don't know what Tom Barr meant, but I think it's important to keep in mind there isn't one magic fertilizer regime that will give you success if you follow it and doom your tank if you deviate from it. There isn't a whole lot of difference between 2 and 3.29 dGH.

#### Deanna

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I’m not quite sure what you are asking in terms of practical application but, perhaps, this info will help you.

To calculate CaCO3 equivalents, we use these formulas:

2.5 x Ca ppm = ppm of CaCO3, which is 37.5ppm CaCO3 equivalent in your 15ppm Ca example.
4.1 x Mg ppm = ppm of CaCO3, which is 20.5ppm CaCO3 equivalent in your 5ppm Mg example.

So, your Ca and Mg example is 58ppm CaCO3 equivalent, but I doubt that this is particularly useful to you as a value.

The German degrees general hardness (dGH, to which we usually refer and our test kits measure) is, in your case, 58ppm CaCO3 equivalent divided by 17.86, which is 3.25 dGH and what your test kit will show.

However, we usually dose in absolute ppm of ions (15ppm Ca and 5ppm Mg in your case) and that is also what is used when we discuss ratios, e.g.; yours is 3:1 Ca:Mg.

#### Savetheplants

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Molar Mass of CaSO4.2H2O is 172.17 g/mol
Molar Mass of Ca is 40.08 g/mol

40.08 g/mol ÷ 172.17 g/mol = 23.28% Ca in CaSO4.2H2O

15 mg/l Ca x 400 liters = 6,000 mg Ca
6,000 mg Ca ÷ 23.28% Ca in CaSO4.2H2O =
25,773 mg CaSO4.H2O2

I know the jar says 22.5% Ca
Maybe the difference has something to do with the ion equivalence you mentioned?

#### Fishworks

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I’m not quite sure what you are asking in terms of practical application but, perhaps, this info will help you.

To calculate CaCO3 equivalents, we use these formulas:

2.5 x Ca ppm = ppm of CaCO3, which is 37.5ppm CaCO3 equivalent in your 15ppm Ca example.
4.1 x Mg ppm = ppm of CaCO3, which is 20.5ppm CaCO3 equivalent in your 5ppm Mg example.

So, your Ca and Mg example is 58ppm CaCO3 equivalent, but I doubt that this is particularly useful to you as a value.

The German degrees general hardness (dGH, to which we usually refer and our test kits measure) is, in your case, 58ppm CaCO3 equivalent divided by 17.86, which is 3.25 dGH and what your test kit will show.

However, we usually dose in absolute ppm of ions (15ppm Ca and 5ppm Mg in your case) and that is also what is used when we discuss ratios, e.g.; yours is 3:1 Ca:Mg.
Thank you, that answers my question, that we normally dose in pure ions equivalent.
Now for my weekly dose after water change, should i dose 3.25dGH (15ppm Ca: 5ppm Mg)?
or weekly 1dGH (4.62ppm Ca: 1.54ppm Mg)? or perhaps 2dGH ( 9.24ppm Ca: 3.08 Mg)?

My water is already 4dGH with 1.6ppm Ca: 7.43ppm Mg at pure ion equivalent.
I think my water is deficient in Ca..

Molar Mass of CaSO4.2H2O is 172.17 g/mol
Molar Mass of Ca is 40.08 g/mol

40.08 g/mol ÷ 172.17 g/mol = 23.28% Ca in CaSO4.2H2O

15 mg/l Ca x 400 liters = 6,000 mg Ca
6,000 mg Ca ÷ 23.28% Ca in CaSO4.2H2O =
25,773 mg CaSO4.H2O2

I know the jar says 22.5% Ca
Maybe the difference has something to do with the ion equivalence you mentioned?

No, probably just margin of error, which is common.
23.28% is close to 22.5%

Thanks

#### Deanna

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Now for my weekly dose after water change, should i dose 3.25dGH (15ppm Ca: 5ppm Mg)?
or weekly 1dGH (4.62ppm Ca: 1.54ppm Mg)? or perhaps 2dGH ( 9.24ppm Ca: 3.08 Mg)?

My water is already 4dGH with 1.6ppm Ca: 7.43ppm Mg at pure ion equivalent.
I think my water is deficient in Ca..
That’s an odd ratio, for municipal water, but I assume that you’ve verified it.

I would, first, decide what you want for the general hardness (dGH) of the tank water. This is a function, mainly, of the animals in your tank. Although I’m not too concerned about a drift of +/- .5 dGH, I target about 4 dGH in my tank.

The generally recommended range for the Ca:Mg ratio is from 2:1 to 4:1. Over the years, I’ve tended more toward a 2:1 ratio of Ca:Mg. So, if I was starting with the Ca and Mg ppm values of your water source, I would not add any Mg, but would add 14ppm Ca. This would achieve my personal preference for a 2:1 Ca:Mg and ~4 dGH.

Now, with water changes and various uptake activities by plants and animals, perhaps some precipitation, etc., your levels are going to change over time. The next challenge, then, is to measure where you are, so you know what to add after a water change. To do this, I’ve found it easier to measure Ca levels than Mg levels with our crude hobby-grade test kits. So long as you have either Ca or Mg, and can measure dGH, you can hearken back to the CaCO3 equivalency formulas to derive the missing ion level.

Precision in any of our tests is virtually impossible to achieve, but I modify the process for using API’s Ca test kit (which is really designed for saltwater tanks) to improve Ca measurements. Use a 50ml sample in order to allow each final reagent drop to equal 2ppm of Ca. The procedure is:
1) Add 10 drops of reagent #1 & mix.
2) Shake reagent #2 for 10 seconds before each test.
3) Add 1 drop at a time of reagent #2 and mix. Multiply the number of drops of reagent #2 by 2 and the result is the ~Ca ppm.

MG - using the GH and Ca results, plug the numbers into the following formula to derive the Mg ppm (note that 1 degree GH = 17.86ppm):
Mg = (GH ppm – 2.5 x Ca ppm) / 4.1

For the modified tests, mentioned above, larger test vials need to be purchased, such as this one: Test Vials

#### Fishworks

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That’s an odd ratio, for municipal water, but I assume that you’ve verified it.

I would, first, decide what you want for the general hardness (dGH) of the tank water. This is a function, mainly, of the animals in your tank. Although I’m not too concerned about a drift of +/- .5 dGH, I target about 4 dGH in my tank.

The generally recommended range for the Ca:Mg ratio is from 2:1 to 4:1. Over the years, I’ve tended more toward a 2:1 ratio of Ca:Mg. So, if I was starting with the Ca and Mg ppm values of your water source, I would not add any Mg, but would add 14ppm Ca. This would achieve my personal preference for a 2:1 Ca:Mg and ~4 dGH.

Now, with water changes and various uptake activities by plants and animals, perhaps some precipitation, etc., your levels are going to change over time. The next challenge, then, is to measure where you are, so you know what to add after a water change. To do this, I’ve found it easier to measure Ca levels than Mg levels with our crude hobby-grade test kits. So long as you have either Ca or Mg, and can measure dGH, you can hearken back to the CaCO3 equivalency formulas to derive the missing ion level.

Precision in any of our tests is virtually impossible to achieve, but I modify the process for using API’s Ca test kit (which is really designed for saltwater tanks) to improve Ca measurements. Use a 50ml sample in order to allow each final reagent drop to equal 2ppm of Ca. The procedure is:
1) Add 10 drops of reagent #1 & mix.
2) Shake reagent #2 for 10 seconds before each test.
3) Add 1 drop at a time of reagent #2 and mix. Multiply the number of drops of reagent #2 by 2 and the result is the ~Ca ppm.

MG - using the GH and Ca results, plug the numbers into the following formula to derive the Mg ppm (note that 1 degree GH = 17.86ppm):
Mg = (GH ppm – 2.5 x Ca ppm) / 4.1

For the modified tests, mentioned above, larger test vials need to be purchased, such as this one: Test Vials

Thanks for putting things into perspective for me.

I spent the last few hours just thinking about my dosage and was leaning towards EI method. I've read that levels Ca 15ppm and Mg 5ppm are good to maintain. So I would dose 15ppm and 5ppm after every water change, that way I am sure I have it, even though my water already contains 7.43ppm of Magnesium. Which takes me to 12.43ppm Magnesium, but the excess magnesium doesn’t matter, as the idea is to provide nutrients for non-limiting growth for my plants.

But now, you got me thinking, using a test kit once a week isn’t so hard. Being able to change and control my weekly dosages, then my stock of dry ferts would last longer.

#### Deanna

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It all depends upon how much time you want to put into it and what your make-up is. I happen to like knowing, a little better, what is going on nutrient-wise and fine tuning dosing. However, your approach with the EI concept and resetting with 50% weekly w/c's is certainly good.

#### Mcnugget

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30g long, low tech, Finnex CRV, medium planted easy plants, rosy tetras, RCS, sterbai corys...
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How do I convert CaCO3 ppm equivalent to Ca and Mg ppm?
Example:
GH 80 ppm as CaCO3
Ca 64 ppm as CaCO3
Ca ppm = Ca as CaCO3 / 2.5
Ca ppm = 64 / 2.5
Ca ppm = 25.6
Mg ppm = (ppm GH as CaCO3 – ppm Ca as CaCO3) / 4.1
Mg ppm = (80 – 64) / 4.1
Mg ppm = 3.9

I would say if you use tap water, to check your city water report. Mine comes in at 20/5, so I omit the Mg from my pps pro kit.

Also, I see Deanna clearly outlined the method already, sorry for the duplicate

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