Mg++ sucking dry rock?

bonefish · Feb 28, 2018

BC32 2.5 months old. Params today: SG: 1.025; NO3: 0; PO4: 0; Alk: 9 dKH; Ca: 460; Mg: 1215.
Other facts: ~50lb dry rock from Florida (a lot I know, but I like it that way). Dosing 4ml @ ESV B-Ionic Alk/Ca for >30d. Currently adjusting Mg with ESV Mg (40mL/d) with ~10ppm increases/dose.

Mg initially tested at 1110 (Salifert) which was why I started dosing Mg. At that point Alk: 9; Ca: 440. Running Reef Crystals, fresh SW tested: Alk: 10; Ca: 420; Mg: 1290. Only a couple small LPS frags at this point so, no serious Mg demand.

Question/theory: Did dry rock in my tank initially absorb Mg from SW, causing the need to supplement? Florida dry rock is mined from inland. Read several articles about how Florida soils are Mg deficient due to acidity. So did I have to "replace" that Mg in the dry rock first, and now supplementation is bringing it up? Or did it go somewhere else? Saw first evidence of coralline algae today...

bonefish · Feb 28, 2018

bonefish said:
BC32 2.5 months old. Params today: SG: 1.025; NO3: 0; PO4: 0; Alk: 9 dKH; Ca: 460; Mg: 1215.
Other facts: ~50lb dry rock from Florida (a lot I know, but I like it that way). Dosing 4ml @ ESV B-Ionic Alk/Ca for >30d. Currently adjusting Mg with ESV Mg (40mL/d) with ~10ppm increases/dose.

Mg initially tested at 1110 (Salifert) which was why I started dosing Mg. At that point Alk: 9; Ca: 440. Running Reef Crystals, fresh SW tested: Alk: 10; Ca: 420; Mg: 1290. Only a couple small LPS frags at this point so, no serious Mg demand.

Question/theory: Did dry rock in my tank initially absorb Mg from SW, causing the need to supplement? Florida dry rock is mined from inland. Read several articles about how Florida soils are Mg deficient due to acidity. So did I have to "replace" that Mg in the dry rock first, and now supplementation is bringing it up? Or did it go somewhere else? Saw first evidence of coralline algae today...

Omission: pH 8.2 Typo: PO4: 0-0.25 (API)

Randy Holmes-Farley · Mar 1, 2018

Magnesium will absorb onto fresh calcium carbonate surfaces.

That said, magnesium test error is also super common.

I'm not sure which yours was, or a mix of the two.

bonefish · Mar 1, 2018

Thanks Randy. Pretty sure the testing is OK its come up consistently and predictably since I began Mg dosing: 1110>1140>1185>1215.

Wonder if that Mg absorption you describe is a contributing factor to why it can take so long to grow coralline algae anywhere in the tank in the presence of dry rock as many people have noted? It seems that if you have a lot of dry rock and never test/dose Mg it could take a while to cover that Mg debt from SW changes alone.

Randy Holmes-Farley · Mar 1, 2018

bonefish said:
Thanks Randy. Pretty sure the testing is OK its come up consistently and predictably since I began Mg dosing: 1110>1140>1185>1215.

Wonder if that Mg absorption you describe is a contributing factor to why it can take so long to grow coralline algae anywhere in the tank in the presence of dry rock as many people have noted? It seems that if you have a lot of dry rock and never test/dose Mg it could take a while to cover that Mg debt from SW changes alone.

That magnesium progression you show doesn't convince me the whole result isn't downshifted from the actual values, and I do not recall seeing anyone report such a drop before. It is also way too big (see below). So I personally think it is not the primary explanation of your result, but it certainly is a process that can happen, depending on what sort of rock you are using. I just do not expect such a massive decline, and can't really see how it is possible.

Many people see coralline begin on plastic surfaces like powerheads and overflows before rock. That suggests the slowness on rock is not low magnesium in the water, but the nature of the surface itself, and what is competing for growth on it.

I discuss magnesium binding to calcium carbonate surfaces here:

http://reefkeeping.com/issues/2006-06/rhf/index.php#21

The magnesium is either replacing calcium in/on the surface (and in that way, a 200 ppm decline in magnesium in the water would cause a 330 ppm increase in calcium in the water) or is taking in carbonate in with it (in which case a 200 ppm drop in magnesium would result in a 46 dKH drop in alkalinity).

It does not appear that either of these two things (330 ppm increase in calcium or a 46 dKH drop in alkalinity) has happened, so I don't believe the magnesium drop (if there was one, I don't know what level your salt mix started at) was from rock binding of magnesium.

bonefish · Mar 1, 2018

Yes, how to know if the Salifert kit, or my use of it, is accurate? I did test the fresh SW mix (Reef Crystals) once and it was 1290, tho it might not be consistent from batch to batch. Prior to testing Mg, and during the period I have been dosing, Alk has been stable at 9 and Ca has been in the 420-460 range.

Thanks for the article I will try to learn something about this! Just off the top, does the intrusion of Mg ions on the surface have anything to do with why the pristine white dry rock changes to a grey color after its been in the tank for a few weeks?

Randy Holmes-Farley · Mar 1, 2018

bonefish said:
Yes, how to know if the Salifert kit, or my use of it, is accurate? I did test the fresh SW mix (Reef Crystals) once and it was 1290, tho it might not be consistent from batch to batch. Prior to testing Mg, and during the period I have been dosing, Alk has been stable at 9 and Ca has been in the 420-460 range.

Thanks for the article I will try to learn something about this! Just off the top, does the intrusion of Mg ions on the surface have anything to do with why the pristine white dry rock changes to a grey color after its been in the tank for a few weeks?

No, the magnesium binding cannot be seen by eye.