Limewater leading to drifting Ca/alk balance

[andy-hipkiss]

Hi Randy,

A Q that I would appreciate your input on if you wouldn't mind?

Although I don't get this myself, it is apparently a fairly common(??) phenomenon if only using limewater that the Ca can be maintained or indeed climb, whilst the alkalinity falls. Any ideas why this would happen in a tank that does not receive any other form of Ca/alk input.

[Randy Holmes-Farley]

Andy:

I've not experienced it myself, but I've heard it claimed. Remember, if we are talking about something that happens over a year or so, even small effects might add up to detectable issues.

Here are some possibilities that could happen with any perfectly balanced calcium and alkalinity additive (limewater, CaCO3/CO2 reactors, two part additives):

1. Conversion of ammonia to nitrate will produce acid that will slowly reduce the alkalinity. If the nitrate is all converted back to N2, then it balances out by taking up acid. If, however, it is removed as nitrate (water changes, etc) then you have a net acid addition over time. The same is true of phosphate: removal of phosphate (except by organisms) will lower the pH.

2. If magnesium were slowly depleted by some incorporation into CaCO3 somewhere in teh system, then alkalinity will deplete relative to calcium.

3. Is fish food a net addition of acid? Not the organics, which balance out, but other things. For example, what is the ph of frozen and/or flake food when put into water? Maybe this is a net addition of acid that over time can be detected.

4. If the top off water is not pure water, but contains some acid (not CO2), then this too might be an issue.

Here's one that might be peculiar to limewater:

5. If the Ca(OH)2 or CaO has a detectable amount of chloride or other anions in it (sulfate, etc) then it will, over time, be a net addition of calcium chloride and raise the calcium over the alkalinity. Specs on the Ca(OH)2 will likely reveal if this is an issue or not for any given grade.

[tatuvaaj]

2. If magnesium were slowly depleted by some incorporation into CaCO3 somewhere in teh system, then alkalinity will deplete relative to calcium.

In "Coral reef ecology" Sorokin writes about red coralline algae: "They deposit in their cell walls a mixture of calcite plus 25% of MgCO3" (ref: Borowitzka MA 1979, Mar Biol 50: 399-347)

With heavy coralline algae growth this might be a good sink for CO3 ?

Edited:
There has also been studies which have shown that at least part of the inorganic carbon in corals comes from the metabolic CO2, meaning that calcium and carbonates are not depleted from the water at the same ratio that "balanced" additives have. True?

[Randy Holmes-Farley]

Tatu:

<< With heavy coralline algae growth this might be a good sink for CO3 ? >>

Yes, it very well could!

<< There has also been studies which have shown that at least part of the inorganic carbon in corals comes from the metabolic CO2, meaning that calcium and carbonates are not depleted from the water at the same ratio that "balanced" additives have. True?>>

They may well get CO2 that way, but they'll still need the alkalinity to convert the CO2 ultimately to CO3--. They can do this simply by pumping out H+ from their cells, but that then lowers the tank alkalinity be the exact amount to form the CaCO3. So I don't think that this pat breaks the balanced need for calcium and alkalinity to the tank overall.

[andy-hipkiss]

Randy / Tatu,

Dunno, people seem to claim a shorter term effect than that.

Anyway, I have asked people on the UK BB I subscribe to, to post here to give you further details. The thread there is UK Reef BB (http://www.ultimatereef.net/cgi-bin/ikonboard/topic.cgi?forum=1&topic=540)

[Randy Holmes-Farley]

Andy:

I read over the posts in that link and I don't have any explanation for a rapid decline except mathematics:

Take the post by Chris:

I've been seeing a steady but gradual decline in my tank's dKH over the last 3 weeks. It went as low as 5.4dKH, whereas before it had usually been approx 9dKH. During this time my Ca level stayed at around the 400 mark.

OK, so alk dropped from 9 dKH to 5.4 dKH (3.2 to 1.9 meq/L). That's a drop of 1.3 meq/L.

Suppose it was simple calcification or other formation of CaCO3.

Then 1.3 meq/L takes out 0.65 mmoles/L of calcium (since you need 2 eq of alk for each eq of calcium).

0.65 mmoles Ca++/L corresponds to 26 ppm of calcium.

So maybe his "around the 400 mark" means that it dropped from 418 ppm to 392 ppm, and he didn't find that drop significant, but the "bigger" drop from 9 to 5.4 dKH did seem significant.


If there is still an issue (at least long term), here's another possibility regarding food:

Calcium comes in with food. Alkalinity does not. So calcium rises over time. I'll see if I can come up with a Ca++ analysis for fishg foods (or maybe someine else has one) and we can calculate the magnitude of this effect.

FWIW, I've only added balanced additives to my tank since I set it up more than 5 years ago. Mostly limewater, and some B-ionic. I don't have an imbalance that I've noticed. I have added a tiny bit of alkalinity in the silicate that I add, but I can't believe that it makes a big difference.

[tatuvaaj]

Andy,

In that thread there was also a theory that limewater precipitates magnesium and doing so "reduces" carbonate alkalinity (I guess the theory is that limewater forms Mg(OH)2 and thus only Ca++ is added to the tank)

Here is an article by Dr. Bingman that shows that this shouldn't happen in aquarium conditions:

"Magnesium Ion Precipitation in Reef Aquaria: A Tempest in a Teapot" http://www.animalnetwork.com/fish2/aqfm/1997/jul/bio/default.asp

[andy-hipkiss]

Thanks Randy

I appreciate your time spent.

I suspect you are right ... Ca tests are not particularly sensitive and 1 meq/l of alk is only 40ppm Ca so a "crash" in alk isn't particularly noticeable on the Ca level.

I wish I could explain the problem (perceived) better, but it does drive me nuts with "engineering" solution (and I don't mean that as a slur against engineers).

The other issue is that European reefers look to the Red Sea for "natural" ca/alk levels ... the Red Sea seems to have 80ppm more Ca than the "normal".

Either way, I've built both "balanced" equations into my Ca/alk balance calculator Balance Calculator (http://www.andy-hipkiss.co.uk/caalkcalc.htm)

[andy-hipkiss]

Tatu,

Thanks ... I wish I could remember more Finnish .... I'm having enough trouble with Dutch at the moment ... and my mind is like a sieve when it comes to languages :(

[simonh]

Randy,

Here was Ron's analysis which includes some numbers for calcium levels in flake foods and brine shrimp:

http://www.animalnetwork.com/fish/data/foods.asp

[Randy Holmes-Farley]

Andy:

<< ... the Red Sea seems to have 80ppm more Ca than the "normal". >>

I also gather it doesn't have much magnesium. Maybe someone has been dripping limewater into the Red Sea:D

<< Ca tests are not particularly sensitive and 1 meq/l of alk is only 40ppm Ca so a "crash" in alk isn't particularly noticeable on the Ca level. >>

It's only 20 ppm, isn't it?

[Randy Holmes-Farley]

Simon:

<< Here was Ron's analysis which includes some numbers for calcium levels in flake foods and brine shrimp: >>

Thanks.

Let's take the Formula foods as typical. At 1000 ppm calcium, they are 0.1% by weight.

I add about 5 g/day to a 90 gallon tank.

That's 0.005 g/day, or 5 mg/day of calcium.

In my tank, that works out to about 5 ppm of extra calcium per year. Maybe it is part of a bigger problem, but it isn't enough to get excited about.

[andy-hipkiss]

Randy,

Duh .. yes ... sorry, 20 ppm.

Interesting that the Red Sea is low(er) in Mg, didn't know that, thanks. Odd that the Ca should be higher than normal in that case, I thought low Mg lead to low Ca. Any idea how come there is this apparent dichotomy?

[Randy Holmes-Farley]

The effect is huge. According to Millero's book "Chemical Oceanography", the Red Sea has almost no magnesium, and more calcium than seawater.

The explanation, according to him, involves the undersea vents that issue hot seawater into the Red Sea. This process takes out the magnesium and releases calcium into the water. Magnesium apparently reacts with hot basalt in water to form MgOHSiO3 which deposits on the bottom.

[andy-hipkiss]

The same is true of phosphate: removal of phosphate (except by organisms) will lower the pH.


Randy,

Would you expand on that please? Is that always true, or say only true for aluminium based PO4 removers.

[LPSman]

Andy,

Do you know if Rowaphos is aluminium based ?

Randy ,

"The same is true of phosphate: removal of phosphate (except by organisms) will lower the pH.

By how much Randy...is it dependent on the amount of PO4 remover used ?

[andy-hipkiss]

Rowaphos is FeOOH (based?). I suspect that the removal compound makes no difference but I await Randy's input.

[Randy Holmes-Farley]

<< The same is true of phosphate: removal of phosphate (except by organisms) will lower the pH. >>

What I meant was this:

Take the oxidation of hypothetical plankton (used in the literature). I'll assume that fish food is chemically similar to plankton:

(CH2O)106(NH3)16(H3PO4) + 138 O2 ----->

106 CO2 + 122 H2O + 16HNO3 + H3PO4

Of course, the H3PO4 is really

H3PO4 ---> 2H+ + HPO4--

or something similar involving PO4---

H3PO4 ---> 3H+ + PO4---

and the nitric acid is really

HNO3 ---> H+ + NO3-

So, if you oxidize plankton, you make acid (both H+ which impacts alkalinity, and CO2, which does not).

If you simply reverse the reaction, as by an algae consuming phosphate, nitrate, CO2, and acid, to grow, then you get right back where you started with the fish food, and there is no net effect on pH or alkalinity.

However, if you tank nitrate or phosphate out of the system without letting it go back into an organic form, then you are left with the H+ from above. Consequently, taking nitrate and/or phosphate away from the tank will result in a net acidification of the tank.


Of course, this ignores exactly how it is being removed. If you just plucked it out with tiny hands, this holds perfectly.

If you take it out by binding to the surface of something, like alumina or iron oxide, whether alkalinity is impacted or not will depend upon whether the "surface" releases anything when the phosphate binds.

Fe+++ + PO4--- ---> FePO4

takes away the phopshate alkalinity from the tank.

Fe(OH)3 + PO4--- -----> FePO4 + 3OH-

does not.

Likewise for alumina:

Al+++ + PO4--- ---> AlPO4

takes away alk

Al(OH)3 + PO4--- ----> AlPO4 + 3OH-

does not

All of these equations are simplifications of what happens at the iron oxide or alumina surfaces, and I'd have to do some more investigating to know what really happens. Likely, the answer won't be clear because it won't be known exactly what sites the iron is binding to, what the sites look like before being put into tank water, and whether anything is being released into the tank water or not.

<< By how much Randy...is it dependent on the amount of PO4 remover used ?>>

I think this effect is small. Note in the first reaction that there are more than a hundred moles of acid from CO2, and 16 moles of acid from nitrate, for each 2 or 3 moles of acid from phosphate. Consequently, it is a much smaller effect than the nitrate from the nitrogen cycle. The impact won't depend on the amount of phosphate remover used (unless the phopshate remover itself add or subtracts alkalinity, which is possible) but it will depend on how much phosphate gets bound to the remover.

[andy-hipkiss]

Randy,

Does the fact that the favourite UK PO4 remover is iron oxyhydroxide (FeOOH) change the equations at all?

[Randy Holmes-Farley]

Andy:

The problem is that the surface layer doesn't necessarily mimic the bulk. Even iron metal is some type of iron oxide on the surface. The exact state will depend on many things, like did it get exposed to any acid or base prior to being added.

I have a book that talks a lot about Fe(OH)3 and related things in water. I'll check it over and see if it tells us anything useful.