Alkalinity or Calcium - Which of these do Corals use more of? [Archive]

that Fish Guy

05/04/2014, 12:15 AM

So the big three are Calcium, Alkalinity, and Magnesium.

I read that Corals use Magnesium the least so you need to dose it less.

So I was wondering of the other two which is used up quicker by Coral - Calcium or Magnesium?

bif24701

05/04/2014, 01:00 AM

Why the question? Because all three need to be controlled. I would say maybe calcium is used somewhat less but why would that be a concern?

ViktorVaughn

05/04/2014, 07:01 AM

Why the question? Because all three need to be controlled. I would say maybe calcium is used somewhat less but why would that be a concern?

I think his question makes perfect sense. When I was learning my tanks needs and consumption of Calcium and Alkalinity I read as many articles as possible to try and find out the relationship between the two while also trying to find out what the consumption rates are for Cal and Alk. I figured it would be handy to know if the rates of use were similar for both or more for either Calcium or Alkalinity so I could plan accordingly in terms of buying bulk 2 part. Didn't really want to buy a bunch of Calcium and a little Alkalinity only to figure out that I needed more of the one I didn't buy.

that Fish Guy

05/04/2014, 07:07 AM

I think his question makes perfect sense. When I was learning my tanks needs and consumption of Calcium and Alkalinity I read as many articles as possible to try and find out the relationship between the two while also trying to find out what the consumption rates are for Cal and Alk. I figured it would be handy to know if the rates of use were similar for both or more for either Calcium or Alkalinity so I could plan accordingly in terms of buying bulk 2 part. Didn't really want to buy a bunch of Calcium and a little Alkalinity only to figure out that I needed more of the one I didn't buy.

Exactly.

Portsie

05/04/2014, 07:11 AM

Only way to know for sure is to test your cal and alk at the same time each day for a few days. You can then average the loss and calculate how much of each your tank is using each day. Pretty sure corals use both at a constant ratio though.

ahmed_iAM

05/04/2014, 07:19 AM

They should theoretically almost be used 1:1. When corals deposit CaCO3 it takes one Ca atoms and one Carbonate or Bicarbonate atom. But often times some extra CO2 gets dissolved by the skimmer or when the water is turned. Hypothetically should be about 1:1 though. I don't know if corals deposits other carbonate molecules by they wouldn't affect the ratio that much

downbeach

05/04/2014, 07:30 AM

"Pure calcium carbonate contains one calcium ion for each carbonate ion. That ratio corresponds to 1 meq/L of alkalinity (= 2.8 dKH or 50 ppm calcium carbonate equivalents) for each 20 ppm of calcium. In a real coral skeleton, some magnesium ions get into spaces ordinarily occupied by calcium, so the real ratio in corals and coralline algae is usually closer to 18-20 ppm calcium for each 1 meq/L of alkalinity, and that ratio varies a bit by organism."

From Randy's article here:

http://reefkeeping.com/issues/2007-04/rhf/index.php#5

So, equal doses of any two part supplements* are designed to provide the same ratio's. Kalkwasser will also provide the same ratio's.

*Assuming your using the same recipe, the same manufacturer, i.e. ESV B-ionic, etc. If you were to mix the recipe's, the manufacturers, you may/will find that equal parts may not work due to differing concentrations between recipe's and manufacturers.

Randy Holmes-Farley

05/04/2014, 07:34 AM

The process we are talking about here is calcification (calcium ion plus carbonate ion gives calcium carbonate). Magnesium gets into the structure in place of the calcium, and the amount of which varies a bit coral to coral (and perhaps tank to tank if the amount of magnesium varies a lot), but generally is about 1/10 of the amount of calcium.

The balanced chemical equation is:

Ca++ + CO3-- --> CaCO3

So the use is about 1:1 in numbers of ions, except the magneisum replacement, as I suggested:

Ca++/Mg++ + CaCO3 --> Ca/MgCO3

From a measurement standpoint, corals use approximately 1 meq/L (2.8 dKH) of alkalinity for each 18 ppm of calcium (pure calcium carboante would give exactly 20 ppm). They also use about 2 ppm of magnesium.

This ratio of calcium to alkalinity is present in a great many of the additive systems we use (CaCO3/CO2 reactors, limewater/kalkwasser; two part systems).

These have more:

When Do Calcium and Alkalinity Demand Not Exactly Balance?
http://reefkeeping.com/issues/2004-12/rhf/index.htm

A Simplified Guide to the Relationship Between Calcium, Alkalinity, Magnesium and pH
http://reefkeeping.com/issues/2006-06/rhf/index.php

dkeller_nc

05/04/2014, 10:51 AM

I figured it would be handy to know if the rates of use were similar for both or more for either Calcium or Alkalinity so I could plan accordingly in terms of buying bulk 2 part. Didn't really want to buy a bunch of Calcium and a little Alkalinity only to figure out that I needed more of the one I didn't buy.

One thing to remember about purchasing the bulk chemicals is that while the calcium and carbonate parts are used in roughly a 1:1 molar ratio, the mass ratio is different (not 1:1).

For example, if you buy sodium carbonate heptahydrate, its molecular formula is Na2CO3-7(H2O), and its molecular weight is 232 grams/mole. If you buy calcium chloride dehydrate, its molecular formula is CaCl2-2(H2O), and its molecular weight is 147 grams/mole.

So if you use the 1:1 molar ratio rule of thumb for consumption of calcium and carbonate, then you'll use a 232/147 = 1.6 ratio of sodium carbonate to calcium chloride by weight. Because most ionic solids that form hydrates also generally have "free water" (not bound to the molecule) in them, one can never be quite that precise when it comes to usage rates, but it's a decent rule of thumb.

ViktorVaughn

05/04/2014, 12:53 PM

One thing to remember about purchasing the bulk chemicals is that while the calcium and carbonate parts are used in roughly a 1:1 molar ratio, the mass ratio is different (not 1:1).

For example, if you buy sodium carbonate heptahydrate, its molecular formula is Na2CO3-7(H2O), and its molecular weight is 232 grams/mole. If you buy calcium chloride dehydrate, its molecular formula is CaCl2-2(H2O), and its molecular weight is 147 grams/mole.

So if you use the 1:1 molar ratio rule of thumb for consumption of calcium and carbonate, then you'll use a 232/147 = 1.6 ratio of sodium carbonate to calcium chloride by weight. Because most ionic solids that form hydrates also generally have "free water" (not bound to the molecule) in them, one can never be quite that precise when it comes to usage rates, but it's a decent rule of thumb.

Thanks for the great, in-depth information. Still kind of confused on the whole subject and trying to figure out what my best dosing regimen will be. Just started a thread in the Reef Chemistry forum to try and clear up some of my questions.

wrott

05/04/2014, 01:59 PM

The only way to dose is to do it, and then test the difference before/after. Kits are just an indicator to see how much has changed--#s are irrelevant. Just keep w/in desired parameters and you'll be ok. It is logical to assume that you will need more alk due to organic acids. It's best to dose smallest amounts possible throughout the day so as to not irritate corals, hence peristaltic pumps. I use a Ca reactor, and test alk every other week to judge params, and observe animals visually to see if any other testing is needed.
I also add Mg and I2 in small amounts--not advised unless you test. But ~20% WC every 2-4 weeks should replenish anything unknown.