Just say NO to magnesium testing: RMM is born

[Reefkeepers Archive]

OK, this thread has been many years in the making, and I'm posting it here since folks in the chem forum are probably already tired of all the threads relating to problematic magnesium testing.

I'm going to propose a method, let's call it the Randy Magnesium Method, or RMM for short. All good methods need a catchy name.

Here it is.

1. NEVER measure magnesium. If it comes free from an ICP, sure, take a look. It's probably fine anyway. Don't buy or use a kit. Chances are the results are not very reliable, and most of the time, if there is a value out of the range of acceptable, it is more likely an error of some sort than a real result.

2. Use a decent salt mix at a decent salinity that has a starting level of magnesium and calcium that you like. If you cannot find one, it is easy to add a fixed amount of magnesium to a salt mix. I did that for many years, and rarely measured the tank itself.

3. ANY time that you add calcium, add 5-10% as much magnesium (so for 10 ppm calcium, add 0.5 - 1 ppm magnesium). If coralline is the main user of alk in your tank, use the 10%, if corals are, use 5%. A lot of products, like commercial two parts, AFR, CaCO3/CO2 reactors with suitable media, and my DIY two part systems all add magnesium for you, without measurement.

4. It will take a very long time for any sort of significant deviation to show, and if you also do water changes, it likely never will.

To some this will sound like a joke, but I think many reefers, especially newer reefers, would be better served by RMM than testing and retesting, dosing and redosing and then getting a new kit and testing again. Just a few minutes ago I finished a thread where a kit change gave a 200 ppm difference in magnesium.

Just say no. Exact magnesium levels are just not that important.

One small issue with this in my system.. the test kit I use has consistently given me the same result over 4 tests, which are 1080, I have started dosing using the seachem magnesium which raises 5ppm for 1 tsp per 20 gal, I do this twice weekly and results have stayed the same (though it did seem like the instructions said that amount will keep mag consistent) and I don't do water changes whatsoever... any thoughts?

 

[reef’r]

What salt mix are you using and at what salinity? What does the kit say on that new salt water?

Salt is instant ocean reef crystals. New batch tested last night at 1.025-1.026 was in between readings on the salifert card, but would equate out to ~1130ppm mag

 

[reef’r]

One small issue with this in my system.. the test kit I use has consistently given me the same result over 4 tests, which are 1080, I have started dosing using the seachem magnesium which raises 5ppm for 1 tsp per 20 gal, I do this twice weekly and results have stayed the same (though it did seem like the instructions said that amount will keep mag consistent) and I don't do water changes whatsoever... any thoughts?

Im obviously no expert as im asking questions in this thread as well, how ever from research I’ve done regarding my own low mag it seems that to raise mag it takes quite a large dose to raise it a noticeable amount

 

[Randy Holmes-Farley]

Salt is instant ocean reef crystals. New batch tested last night at 1.025-1.026 was in between readings on the salifert card, but would equate out to ~1130ppm mag

Thus, your aquarium matches fresh RC. IMO, RC has plenty of magnesium at that specific gravity and I'd ignore the readings for now. Going forward, I'd just add magnesium when you add calcium.

If you want to have higher magnesium than RC provides, I'd just add it to the new salt water at a fixed amount (say, 50 or 100 ppm) and still not bother with testing.

 

[Randy Holmes-Farley]

Im obviously no expert as im asking questions in this thread as well, how ever from research I’ve done regarding my own low mag it seems that to raise mag it takes quite a large dose to raise it a noticeable amount

Yes, it takes a huge amount. Here's a calculator:

Reef Chemistry Calculator

 

[Reefkeepers Archive]

Yes, it takes a huge amount. Here's a calculator:

Reef Chemistry Calculator

I had a feeling I wasn't using enough, thanks!

 

[mikst]

3. ANY time that you add calcium, add 5-10% as much magnesium (so for 10 ppm calcium, add 0.5 - 1 ppm magnesium). If coralline is the main user of alk in your tank, use the 10%, if corals are, use 5%.

I like math. Chemistry is in many ways math, but I am somehow bad at translating units or something because I can't get it to work.

I was reading your diy 2/3 part solutions that you mention brs uses (https://reefkeeping.com/issues/2006-02/rhf/index.php#6). I'm looking at part 1 and part 3 since we're just talking about calcium and magnesium for the RMM.
For recipe 1, you make a gallon of part 1. In this gallon, there is 37,000 ppm of calcium ions.
In part 3 (magnesium dose) you make up a gallon of solute that has 47,000 ppm magnesium ions.

After a gallon of part 1 (also part 2) is consumed, dose 610mL of part 3.

What I get is that 140,045 mg (37,000 mg/L times 3.785 L) Ca ions are dosed along with 28,670 mg (47,000 mg/L times 0.610 L) Mg ions.

But that comes down to 20 percent. Not 5 to 10. What am I doing wrong?

Thanks in advance.

 

[killer2001]

I do daily water changes (2%) and dose AFR and never had an issue with Mg levels (according to multiple ICP tests). Potassium on the other hand... now that's a different story.

 

[Randy Holmes-Farley]

I like math. Chemistry is in many ways math, but I am somehow bad at translating units or something because I can't get it to work.

I was reading your diy 2/3 part solutions that you mention brs uses (https://reefkeeping.com/issues/2006-02/rhf/index.php#6). I'm looking at part 1 and part 3 since we're just talking about calcium and magnesium for the RMM.
For recipe 1, you make a gallon of part 1. In this gallon, there is 37,000 ppm of calcium ions.
In part 3 (magnesium dose) you make up a gallon of solute that has 47,000 ppm magnesium ions.

After a gallon of part 1 (also part 2) is consumed, dose 610mL of part 3.

What I get is that 140,045 mg (37,000 mg/L times 3.785 L) Ca ions are dosed along with 28,670 mg (47,000 mg/L times 0.610 L) Mg ions.

But that comes down to 20 percent. Not 5 to 10. What am I doing wrong?

Thanks in advance.

You aren’t doing anything wrong, just not u standing the whole picture. There are two reasons magnesium is added in that context. One is to offset consumption, and one is to offset the magnesium decline that comes from the salinity rise due to accumulating sodium and chloride. When that salinity rise is corrected by lowering salinity, magnesium and everything else gets pushed down, so magnesium must be added to offset that effect.

That second portion is not part of RMM because many methods do not require it.

I go through the exact math of how I determined both parts in this article:


An Improved Do-it-Yourself Two-Part Calcium and Alkalinity Supplement System by Randy Holmes-Farley - Reefkeeping.com

From it:

After adding 594 grams of baking soda (1 gallon of Recipe #1), we will have added 163 grams of sodium. In natural seawater, magnesium is present at about 12.0% of the sodium concentration (by weight). In order to match the magnesium additions to the sodium additions to leave them in a natural ratio, we need to add 12% of 163 grams, or 19.5 grams, of magnesium for every gallon of the two-part additive that we add.

Additionally, we may want to account for magnesium that is actually incorporated into the coral skeletons. For this calculation, I have assumed that the amount of magnesium incorporated is about 6.5% of the calcium level (by weight), or about 2.5% of the skeleton by weight. In the course of adding this gallon of both parts of the two part supplement, we added 141 grams of calcium, so we need to add 0.065 x 141 = 9 grams of magnesium to account for this deposition.

The magnesium parts of the recipe are designed to add enough magnesium so that it is not depleted by either of the two means described above. Because the magnesium supplement (either version) is 47,000 mg/L in magnesium, we need to add (9 +19.5) grams/47 g/L = 610 ml of the magnesium solution for each gallon of the other parts of Recipe #1.

 

[mikst]

You aren’t doing anything wrong, just not u standing the whole picture. There are two reasons magnesium is added in that context.

Oh! Well that makes perfect sense. I will hit that article next. I'm still working on this one (https://www.reefedition.com/the-many-methods-for-supplementing-calcium-and-alkalinity/)

So, I intend to use kalkwasser, not 2 part. should I then only be dosing magnesium ions in proportion to the calcium/magnesium consumption ratio described above?

Thats what it sounds like to me.

 

[Randy Holmes-Farley]

Oh! Well that makes perfect sense. I will hit that article next. I'm still working on this one (https://www.reefedition.com/the-many-methods-for-supplementing-calcium-and-alkalinity/)

So, I intend to use kalkwasser, not 2 part. should I then only be dosing magnesium ions in proportion to the calcium/magnesium consumption ratio described above?

Thats what it sounds like to me.

Yes. More to the higher magnesium dosing side side if coralline is the biggest consumer of alk and calcium, and more to the lower magnesium dosing side if it is not.