magnesium dosing

yme

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I was just wondering....

I perform a 50% waterchange each week. The "fresh" water contains 7.5 mg/l Mg. So I add 3.75 mg/l Mg per week (apart from the Mg in TMG). This results than in a maximum of again 7.5 mg/l Mg. From what I read on the forum, 7.5 mg/l Mg should be enough.
But is it possible that plants use so much Mg that Mg becomes limiting using my regime?

greets,

yme
 

Tom Barr

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I think it's really doubtful that they will use more than this to the point it becomes limiting, even under high light, loads of CO2 and a packed tank full of fast growing stem plants.

If you have Anubias coffeefolia, this plant seemed to express Mg deficiency the best in all the species I worked with, this also means growing it not in the sediment, attached to wood etc. It will have banding on the leaves(yellowing) along the ribs.

Takes some time for it to occur though.

In general plant tissue, the difference between N and Mg is about 12-15 X.

So I suppose even if you used a larger ratio or a smaller, it would take a huge difference to cause a limiting factor for Mg.

Say the N-NO3 is 5ppm(22 ppm of NO3), this means you'd might get some limiting if you maintained less than say 0.5ppm of Mg.

I think it's unlike as you have 10X this amount.

On the upper bound, I used to have tap that had 50ppm of Mg.
I grew most species with issue, but if was pretty nasty tap overall for plants/fish, mostly due to extremely high KH as well.

This said, you can always add a bit more MgSO4 and see.
It does not take much Mg, and generally it's a rare deficiency(I have never met anyone that's demostrated Mg limitation other than a few people in the USA in the Bay area back in the 1990's who did not dose any GH/Mg/Ca etc). I had not been adding it for sometime under high light before I got weird growth coloration in some Anubias.

Even there, it was rare.

Adding the MgSO4 took care of the veining rather quick(2-3 weeks).

Regards,
Tom Barr
 

yme

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good to know :)

I don't have that anubias and probably no Mg deficiency (don't see any green veins, yellow leaves), but one starts wondering now and then. :D

thanks,

yme
 

Tom Barr

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Simply doing replacement dosing based on the typical aquatic plant nutrient ratio takes care of it. Other than few metal traces, the ranges of most nutrients is really wide, so over dosing is fairly hard to do.

Something like copper you have to be careful with, or CO2...........
Ca, Mg, K+, NO3, pretty wide ranges.......

I add more GH if the fish are sick etc, as they are salts.
Rather than NaCL etc.


Regards,
Tom Barr
 

yme

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didn't know that about higher GH instead of NaCl. makes sense!

anyway, I browsed on the internet and found that hannah sells 2 Mg fotometers.

the first is a single parameter Mg fotometer that can determine Mg concentrations pretty accurate, but only low levels. I read the manual, but is noy clear to me whether Ca will interfere with the measurement.

the second is a Mg and Ca meter wich measures at higher levels but less accurate.

do you have some more info? do you think you can more or less accurate measure Mg or Ca with these fotometers? (especially whether Ca inteferes with the Mg measurement)


greets,

yme

ps: no I do not have too much money, just gathering info :D
 

Tom Barr

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A Ca++ test kit works fine, since the GH is the total sum of both Ca++ and Mg++, you take that and use the difference for Mg.

Lamotte etc , most Calcium test kits etc are fine and accurate for our purposes.

Most do as CaCO3, so the CO3 needs removed for Ca++, some kits have it built it for that conversion already, others give you a conversion factor.

Regards,
Tom Barr
 

yme

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yeh...

I have done those tests in the past. I was never completely convinced that it was accurate: 2 measurements, each with some error. do the math, but will the result really reflect the actual Mg concentration? isný it possible that the math will give you 10 mg/l while you have 5 mg/l in the tank? for some people, that's a biog difference.

I must say, I never did proper experiments regarding this....

greets,

yme
 

Tom Barr

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You can always make a reference solution for Mg++, and Ca++, then make a mix of both say 5 ppm Ca++, 2ppm of Mg, then another of 4ppm of Mg, and 15ppm of Ca++.

Then also keep your regular reference solutions, say 50ppm of Mg++, and 100ppm of Ca++, and use dilutions to make your standards, say 1ppm, 5ppm, 10ppm, etc for individual cations.

That would be the best way to see and test.

I'd get a decent low range kit, you can use the photometers if you wish, or a Hach titrator which are very accurate for those cations.

You can also use RO and add known weights of Mg/Ca and see that way if you do several larger water changes, clean good, get the aquarium doing really well etc.

Several back to back large 80% water changes do this pretty well, particularly when done in the morning a hour after the lights have come on for each water change.

After a few days of this, the % left is not much, it's pretty much 95% all whatever you have dosed.

Regards,
Tom Barr