Calcium - Alkalinity - pH


Active Member
For what it's worth.
Regarding this hobby and our saltwater aquariums, whenever you think of the following - think of the following.
What I mean is this.
If your calcium is low - your alkalinity is most likely too high.
If your calcium is too high - your alkalinity is most likely too low.
If you alkalinityis too low - your calcium is probably too high.
If your alkalinity is too high - your calcium is probably too low.
I'm done now :)


Active Member
Thanx Broomer.
That puts it in a bit better light for understanding. Any time you think of little things like that............feel free to blurt them out.

nm reef

Active Member
Brommer....hide....somebody just called for the guys with the funny white jacket.
Been a lot of those calcium/alkalinity/ph questions of late huh.....:cool:
Excuse my friend above for venting a bit........happens to all dedicated reef keepers now and then.......
When the urge hits me I take a 6 pack of Bud and hide in the closet......;)


Active Member
Well broomer I got a question for you....I dont have an alkalinity test kit.....I do however test both calcium and ph. My calcium is somewhere in the vacinity of 450-500, and my pH is 8.4. Where would you imagine my alkalinity would be? I"m going to buy a test kit for it on my next pay day..
Is 8.3 a more ideal pH then 8.4 or does it matter?
Should my calcium level be <450?
How would my alk relate to both of them because they both seem (to me anyway) to be at pretty decent levels....

Active Member
broomer the more you vent the more i learn so vent as much as you like. thanks for the help in my post i am getting a alkalinity test kit tomorrow. Would you like to vent on lighting wright now i have about 2.56watts/gal and i would like to get some coral some one told me the 5 watts/gal would be good so vent and tell me what you think.

reef dude

Ok, my calcium is 350, my alk is 3.5 - 4.0 meq/L and my pH is 8.3
and i cannot raise my calcium above 350!!! i dose kalk all the time!
what do i do? Super dKH buffer????


Active Member
kalk, is a good way to maintain, IMO, but not a good way to increase calcium.
get some liquid calcium, we use and like seachem, if not, get some kent, they are both good and will help in increasing your calcium better than kalk.


Active Member
I think I swallowed some saltwater yesterday repriming my siphon tubes after cleaning. Sort of went to me head :rolleyes:
I would hesitate to guess where your alkalinity is Azonic, without doing an actual test on your tankwater. The calcium/alkalinity levels do follow an inverse relationship to each other.
One up the other down.
pH on the other hand is not so simple - although there is a direct relationship - it can go either way depending on some other factors in the tank.
When we mix up our saltmix and freshwater - we have a starting point. Whatever that starting point is ..... may be different between our tanks - but it's still OUR starting point.
My point is this:
When trying to adjust one level only - you will alter the others.
Adding just buffer is unwise.
Adding just calcium is unwise.
Using only a product like pH up is unwise.
When measuring pH - think of this as a snapshot in time.
The minute you do the test - is the pH level of that sample at that point in time. The pH moves around often - and over time .... want's to drop in most every case.
It will remain somewhat steady within a RANGE if there's a decent amount of carbonates in the tank. Decent meaning enough carbonates to "buffer" or resist the natural tendency for pH to drop. But it will still swing up and down in a tank that contains green plants/algae that is lighted during the day, and dark at night. It will also swing if you have fish.
It will also swing if you vary the circulation within the tank.
CO2 gas is the other player in this mind bender - it pulls the pH down - but it also is a source of CARBON .... which will affect the amount of CARBONATES in the tankwater.
As the pH trys to drop - the alkalinity will "buffer" or try to keep it within some range. That range can be higher than desired, or lower than desired.
Here's a basic equation that shows a relationship.
CO2+H2O <~~> H2CO3 <~~>(H+)+(HCO3-) <~~> 2(H+)+(CO3--)
CO2 = carbon dioxide
H2O - water
H2CO3 = carbonic acid
(H+) = one hydrogen
HCO3 = bicarbonate
2(H+) = two hydrogens
(CO3--) = carbonate
At any given time in the tank - these elements and compounds are changing. As CO2 gas enters the tank from the fish breathing and from the gas exchange at the surface of the tank - it tends to drive the equation to the right ~~> and will join with water molecule to form carbonic acid. Most all acids contain hydrogen. pH is a measurement of the amount of H+ hydrogen in the tankwater at a given snapshot in time.
You see the H+ as the pH lowering culprit. Too many and the pH drops. We add buffers ( carbonates ) to the tank to offset these H+ hydrogen ions. Carbonates will "neutralize" the H+ and drive the equation back to the left <~~
The THREE main players in the tankwater chemistry are;
Water is hydrogen and oxygen
Carbon Dioxide is carbon and oxygen
Carbonic Acid is all 3, hydrogen, carbon and oxygen
Bicarbonate is another mix of all three - in a different form.
Carbonate is just carbon and oxygen and frees up the hydrogen.
Calcium and Magnesium ( as well as most all the other ions and compounds ) are affected by these 3 main players.
It's like opening a new box of colored TinkerToys. Remember those ?
You have so many colored wooden dowel rods, so many connectors. You can build stuff out of these - but can only add so many to the connectors - there's only 4 holes in the round connectors.
Some connectors may have 4 holes, some may have 3 holes, some have only 2 holes.
When we add more colored dowel rods in the form of calcium or buffers - we can build some more stuff. But .... you may be limited to what you can build. If you add more dowels than you have connectors - you will end up with spare colored dowel rods.
You may end up with too many alkalinity carbonates.
You may end up with too many calciums.
You may end up with too may hydrogens.
If you want to use all the pieces, and not have too many leftovers - you must add the appropriate colored rods and connectors ...... so they all can fit together.
The thing I'm trying to say - without getting all wrapped up in the actual equations is this.
Buy some test kits and use them.
Monitor pH
Monitor alkalinity
Monitor calcium
Write the results down each time you test.
As you dose additives for buffer carbonate, or pH or calcium ... just realize that when doing so ... you are tossing things into the equation that will change the other parameters.
Dosing only calcium will mess up your alkalinity.
Dosing only buffers will mess up your calcium.
Both may move your pH - or not.
It's a balancing act, and if one is out of balance - chances are very good that the other's are out of balance as well.
It's like mixing colored dyes or paint.
Say you have an original color of paint.
If you have some blue in the paint, and want the paint to look more yellow - you can add some yellow.
But watch out - what you will get is green.
If you get too much green - and add more blue - it may not go back to the original blue. It will look blue-green.
If you keep adding paint colors - before you know it - you may have messed up the color so bad that the only way to correct it is to start over with some new original colored paint.
Water changes allow you to start fresh.
Water changes allows you to get things back into balance.
If your tankwater chemistry is whacked out - because you've dosed incorrectly - then it may be time to start fresh.
If the tankwater is really whacked out ....
you may have thrown in so far out of balance - that you've made black paint.
Fish and inverts don't like black paint.
Fish and inverts don't like water that is whacked out.
Fish and inverts like saltwater that is balanced.
Time for a series of water changes to start new again.
Then begin a routine of adding the appropriate amounts of all of the chemicals to keep things in better balance.
Not too many spare TinkerToy parts floating around in there.
Not too much yellow, green, blue or black paint.
Each tank's different - but the tankwater chemistry is somewhat the same. It's only the levels that are different between tanks.
The relationships are the same for all of us.


Active Member
crazy? i was crazzy once. they locked me up in a rubber room with rats. rats? i hate rats they drive me crazy! crazy? i was crazy once. they locked me up in a rubber room with rats. rats? i hate rats. they drive me crazy! crazy?
and so on
but seriously this post should be archived for all teh ca alk ph q's in the future.
one other q. bang guy had stated in a recnet post that the addition of an acid, not sure of the name, could be utilized to bring down ph, and alk levels that were out of whack. any thoughts on this? i would be leary of adding any kind of acid to the tank, even a weak one. too many bad things can occur1
good luck


Active Member
most likely acetic acid which can be found in white vinegar along with water.
I don't add this to my tanks - but some do.
Another compound containing carbon, oxygen and hydrogen.
Adding to the tankwater - the molecule bonds will break - freeing up the H+ as acid to drop pH, leaving behind the other carbon and oxygens to form other compounds.


Active Member
i found it interesting as in all four years of this hobby i have never heard anyone suggest adding acid to the tank, however increasing the h+ ions never really seems to be the prob, its always that the water is already too acidic/neutral and reefers nee to get it to the more basic side of 8.3-8.4
white vinegar. id be quite sketchy about adding this to the tank
next sem. im taking organic chem, so im sure these questions will arise, or i will make them!
good luck


Active Member
I find these relationships to be one of the most interesting parts of the hobby.
Not the funnest - not like watching the tank - that is by far the most interesting.
This tankwater chemistry just happens to be the most puzzling part for me - one that I continuously struggle to comprehend and fully understand.
I'm still trying to figure it all out - someday I might.
Until then - I keep testing the saltwater and writing down the results - and writing down how much and when I dose additives.
I've got the balance figured out dose wise - it repeats and is fairly predictable. This may be as good as it gets for me - but again - it sure would be nice to sit down someday with a marine chemist - and talk.


Active Member
well thank you for doing so because im sure it has educated quite a few folk on this here board!
good luck
Can this even be done on this board?
Yup.....and I did a few days ago....interesting thread and deserves to ride on top for a while...bdwalley
Good thread.... I disagree with the notion that adding just calcium is unwise and will mess up alkalinity though.
For those reefs that are heavily stocked, calcium untake by the corals is far greater than say a FOWLR setup. At any given time an ideal balance in water chemistry, lighting etc can also cause a huge (far greater than normal) uptake in a very short time.
A couple of months ago Magnesium dropped from 1320 to 900 and Calcium fell from 460 to 300 !!! Alkalinity and PH remained stable +/- 5% (normal) even with a calcium reactor keeping things going it was necessary to add additonal liquid calcium to get the levels back up. The extra calcium did nothing to the PH or Alkalinity
While I would agree that buffering water isn't ideally what you want to be forced to do daily sometimes it is necessary.
IMHO monitoring and logging levels with a good test kit (to build a database from which these "events" might be predicted) is something anyone can do and everyone should.
No offense meant , no harm intended.
As for speaking to a chemist try Dr. Randy Holmes-Farley. Dr Farley has answered questions from hobbiests about reef chemisty for years. If you can't find a point of contact I can e-mail you the info.
Here is an excert from one of Dr. Farley's articles:

Acute Upward pH Spikes
In an acute high pH situation (such as 5 gallons of limewater entering the tank and sending the pH over 9), adding an acid such as vinegar, muriatic acid (HCl or hydrochloric acid) or sulfuric acid are all acceptable ways to lower the pH. A number of aquarists have successfully treated such situations with vinegar, and have not ended up losing any animals, though the tank is loaded with white calcium carbonate precipitate. I’ve added HCl to my tank in similar situations without difficulty.
If you do such an acid treatment, be very careful to not overshoot, and monitor the pH during any acid additions. I would only intervene in this fashion if I could monitor the pH in real time, and could add the acid to a high flow situation far from any organism. Diluting the acid in water (say, 20:1 or 100:1) prior to adding it to the tank is highly recommended both for your safety and that of the tank inhabitants (dilution isn’t necessary for vinegar which is already dilute).



Active Member
slowest is fastest
By all means ... No offense taken and I know no harm was intended.
You make a very good point - one that has also kept me wondering. That is, what is the role that magnesium plays in our tank and waterchange water - and how does it come into play.
I believe I understand that having insufficient magnesium, less than the 1300 ppm found naturally - can prevent us from keeping our calcium levels up into the 400ppm+ range.
This I've read before - but have little direct experience.
My statement that adding just calcium will mess up your alkalinity - is true if, and only if, everything else is somewhat constant, stable and holding at a known level. I believe that this includes magnesium too.
I try to avoid using the phrase "all things being equal" that we've read so often. IMO - this phrase is one of the most dangerous presumptions we can make - because as you know - nothing is equal and everything changes in the tankwater - given enough time and load.
Your's is such a great case to mention - because it shows how a change in one - changes another. In your case - a significant drop in magnesium, allowed your calcium level to max out down around 300 - even with the reactor running. The lower magnesium level "prevented" the calcium from being saturated up in the 400+ range.
Another piece of the puzzle.
Were you able to determine what the loss of magnesium was due to ?
Did you change saltmixes or do a substantial water change, or anything else out of the ordinary, or do you think that the drop in magnesium was or is a natural occurance in our tanks ?
I'm always eager to hear of other's experiences such as yours, especially on this topic. It interests me. I certainly don't have all the answers - and like many - I am trying to learn too.
Helps us to figure it out on our own tanks - on a friendly, accessible level.
I have yet to figure out what happened. I will share a couple of piccs though. It is what made me realize something wasn't right. The small devils hand is normally a brown/mustard color. this picture is not doctored but shows the huge change in color. The second devils hand was also a normal brown beige color.
I am not sure if the Magnesium was used first and caused the calcium to fall or if everything happened all at once. I do know that the color morph happened over the space of about 12 hours. Nothing out of the ordinary was done or happened apart from the Ca, Mg drop that I am aware of (ie lights, water change, new addions etc) I believe everthing hit exactly right (or wrong but I like being an optimist) and some much needed nutrients were stripped from the water all at once. There were several other color changes as well which leads me to believe that most if not all corals were affected.
Anyway here are the pics (at least I hope this works)

i like fish

I was constanly trying to get calcium and alkalinity up till my LFS guy mentioned magnesium. I bought a kit and sure enough, magnesium was at 1200. I brought it up to 1320 and now calcium and alkalinity are right where I want them, I even had to back off my dosages of calcium and alkalinity supplements.