Balancing calcium and alkalinity with magnesium

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Yas

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Today, I watched a very interesting video that @Algaeworrier shared in a thread. It was super fantastic, but there were two things that I couldn't understand. Could you help?


At around 27:55, he says magnesium is bonded to carbonates. So carbonates can show up on a test kit, but it's not available to make calcium carbonates.

Now I understand why magnesium can raise both calcium and alkalinity (carbonates) although those two are on a teeter-totter relation. But, as he states, with high magnesium, even if alkalinity is kept high on a test kit, such alkalinity already bound with magnesium cannot be used for crystallization.

I dose alkalinity, calcium, and magnesium, and everything seems to be balancing well on test kits. However, if his explanation is correct (I'm sure it is), it means my alkalinity is always less than it reads on a test kit and not fully consumed for crystallization?

Here's the other question. At around 30:50, he explains coral's mechanism in relation to nitrates and phosphates. He says corals can live with zero nitrates while they cannot live with zero phosphate. I do not understand this. Because he says corals have excellent mechanism to take nitrates out of water column. I don't understand corals can live with zero nitrates though they have good ability to process it. Does it simply mean that "they can" doesn't necessarily mean "they must (consume)?"

I would appreciate any input. Thank you!
 
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Randy Holmes-Farley

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On the nitrate question, there is no doubt that corals can thrive at 0.1 ppm nitrate (zero on most hobby kits) since that is the level in the ocean. I do not agree that very low nitrate is a good goal for reef tanks for several reasons, including lack of other good sources in reef tanks, and the potential for dinos at low nutrient levels.

Corals may get their N from other sources, not just nitrate. Amino acids, ammonia, consumed particulate foods, etc. That doesn't mean you can drop nitrate very low unless you ensure these other sources are adequately present, but in the ocean, nitrate can be below 1 ppm and corals do fine.

On the magnesium, it is true that all metals will form transient soluble complexes with carbonate and bicarbonate, and magnesium has the largest fraction of those ions bound this way, partly due to its chemical nature, and partly due to the fact that there is so much of it present. While it may be possible that the bioavailability of bicarbonate (or carbonate) to corals may be reduced in this fashion, I do not think it is something to think about unless you are intentionally trying to drive magnesium to unusually high or low levels.

I think this is a fairly esoteric discussion for interested chemists, not anything that has big implications for reef hobbyists. Thousands of hobbyists have effectively explored a wide range of chemical properties to maintain reef aquaria, and the results do not suggest that slightly elevated magnesium is any issue.
 
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Yas

Yas

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Thank you so much, Randy! As always, you are the man!

It's a relief to know actual hobby tanks are not that cut-and-dried, and various factors are involved. I was thinking "do I have to strictly follow this guideline? But how can I make seemingly-balanced minerals really-balanced minerals?" You are totally right that thousands of hobbyists are doing with a wide range of chemical properties, and I don't have to be overly nervous!

Thank you very much!
 

Randy Holmes-Farley

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Thank you so much, Randy! As always, you are the man!

It's a relief to know actual hobby tanks are not that cut-and-dried, and various factors are involved. I was thinking "do I have to strictly follow this guideline? But how can I make seemingly-balanced minerals really-balanced minerals?" You are totally right that thousands of hobbyists are doing with a wide range of chemical properties, and I don't have to be overly nervous!

Thank you very much!

You're welcome.

Happy reefing. :)
 
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