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Old 01-12-2009, 10:02 PM
Myka's Avatar
Myka Myka is offline
Join Date: Mar 2004
Location: Saskatoon, SK.
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Default Myka's Guide to Calcium, Alkalinity, Magnesium, pH, and Salinity

I posted this article up on some other forums a few years ago, and have had a couple people ask me to post it up here on SaskReef. I hope it can help some of you out.

Calcium, alkalinity, and magnesium are the backbone of your reef. This Guide will explore the relationship between these three elements (and a couple others) by simplifying the information, and presenting it in an easy to read and understand short format that hopefully won’t bore you too much. I would also like to point out that even though some seasoned reefers may pick up a tid bit of info here and there in this guide, it is mainly aimed at novice reefers to give them a good starting point.


~ The Importance of Test Kits
~ Calcium
~ Alkalinity
~ Balanced Calcium and Alkalinity
~ Magnesium
~ pH
~ Waterchanges , Salinity, Specific Gravity, and Temperature
~ Dosing

The Importance of Test Kits

Good quality test kits are very important, and I would suggest you spend the extra few bucks on at least a few good quality kits for your reef tank. The most important would be calcium, alkalinity, magnesium, and nitrate; for which I would suggest the use of Salifert at least. I by far prefer Elos for calcium and alkalinity, but I find Salifert just as good as Elos for magnesium. The new Hanna Alkalinity Checker is getting great reviews as well although I haven't tried it myself yet.

pH test kits aren't overly accurate, so I would suggest you either don't worry about it (explained later) or buy a hand held pH meter (about $60) or pH monitor (about $120). pH meters and monitors are notorious for burning out probes. You can expect the probe on either to last a couple years, any more than that and you're a lucky one!

An ammonia kit is a great indicator like the nitrite kit, and imo is necessary when cycling your aquarium, and a handy tool to use for figuring out what's going on when things are amiss. I prefer Nessler test kits as opposed to salicylate test kits for ammonia. Nessler kits read only toxic ammonia (NH3) while salicylate kits test both toxic ammonia and ammonium (NH4) which is not toxic for your reef. If you ever have the need to use an ammonia detoxifier like AmQuel or Prime you will need a Nessler kit to accurate test the ammonia after using those products. Those products will produce a "false" positive on a salicylate kit. API used to offer both a NH3 kit and a NH3+NH4 kit, but now oly offers the salicylate kit. The only Nessler kit on the market that I know of that is easy to obtain is the SeaChem Ammonia kit. You can also use SeaChem Ammonia Alerts.

Nitrite isn't overly important in a saltwater aquarium because nitrite is not toxic in saltwater the way it is in freshwater so you don't really need a nitrite kit. Nitrite very rarely gets to a high enough level in an aquarium that it is even vaguely toxic (think a hundred ppm). There is little need for a nitrite kit, but it is a great indicator as to where you are in your cycle when first starting your aquarium, and in mature tanks it is an indicator that something is amiss.

Nitrate is another important nutrient in the tank. I would suggest either Salifert or Elos. Using either of these kits, nitrate should be no higher than 5 ppm, but essentially it should be undectable on these kits.

For phosphate the Salifert or Elos regular sensitivity are both fine. When using these kits phosphate should be completely undetectable or it is too high. For those more serious reefers you should consider the D-D Merck high sensitivity Phosphate kit (about $80) or the Elos High Sensitivity Phosphate kit. Phosphate would be the number one cause of brown corals and algae growth, with nitrate as a close second. The new Hanna Phosphate checker is also receiving great reviews although I have not used it myself yet.


Calcium is used by all corals with a skeleton (SPS and LPS), clams, calcareous algaes (coralline, halimeda), and even invertebrate to build their shells. Generally, natural seawater contains a calcium level of 410 ppm [as per Randy Holmes-Farley]. A calcium reading of lower than 360 ppm can compromise coral growth. Having more than 450 ppm calcium is a waste of additives, and if you take a look below at the balanced ratio to alkalinity, a high calcium level requires a very high alkalinity level! Studies have shown that an elevated calcium level does not increase corals’ growth rate or density.

If you dose too much calcium in one go while your alkalinity is too low you can cause the calcium to precipitate out which looks like a snow storm in your tank, and instantly pulls a bunch of calcium and alkalinity out of your water. You will have to re-dose. When you dose calcium, the alkalinity level will drop slightly. I like to keep my own tank’s calcium between 415-425 ppm.

I prefer to dose calcium before alkalinity (provided alkalinity isn't lower than 6 dKH, otherwise dose alkalinity first), and magnesium before both of these. Dose in a high flow area, and make sure the slurry is diluted before it touches any corals as the slurry will immediately burn corals.


Alkalinity is what we use to measure bicarbonate as test kits for bicarbonate would be difficult and expensive to obtain. Alkalinity in (very) short is salt water’s ability to buffer the pH against acids which try to lower the pH. Generally, natural seawater contains an alkalinity level of 7 dKH. Corals use calcium and alkalinity in equal parts to create their skeletons, which works out to be 2.8 dKH for every 20 ppm of calcium.

Low alkalinity can be one of the factors of nuisance algae growth. Both low and high alkalinity levels can cause precipitation of calcium. Low alkalinity causes precipitation in the water, while high alkalinity causes precipitation on heat sources like heaters and powerhead impellers.

When you dose alkalinity you will notice that it affects your calcium level by slightly decreasing it. When alkalinity is allowed to fall you will notice that your calcium will rise slightly in comparison causing an imbalance. Alkalinity will be the most challenging to keep consistent. Check it often, and dose as required; this will also help to keep your pH steady. To match the calcium level of 410-420 ppm I like to keep in my tank, I like to keep my alkalinity at about 7-8 dKH.

Balanced Calcium and Alkalinity: (as per Randy Holmes-Farley)

360 ppm to 0 dKH
370 ppm to 1.4 dKH
380 ppm to 2.8 dKH
390 ppm to 4.2 dKH
400 ppm to 5.6 dKH
410 ppm to 7 dKH (natural seawater)
420 ppm to 8.4 dKH
430 ppm to 9.8 dKH
440 ppm to 11.2 dKH
450 ppm to 12.6 dKH
460 ppm to 14 dKH

The relationship between calcium and alkalinity is an important one, and the numbers need to be balanced for optimal absorption by the corals and other life much like the human body needs a balance between calcium and phosphorus to make good bones. Some people may experiment with elevating the alkalinity level beyond the balanced ratio to try to increase coral growth, but there is no proof of this theory. Keep an eye on your calcium level though if you experiment like this as it may decrease your calcium, and may make it difficult to keep your calcium at an acceptable level. Also keep an eye on the precipitation on heat sources as this becomes a waste of your calcium and alkalinity dosing efforts. My aquarium’s alkalinity shot up to 18 dKH at one time, and at this point some corals did show irritation, but nothing was lost.


Magnesium both helps to buffer the pH from falling, and reduces the precipitation of calcium. Think of magnesium as the fulcrum of a teeter totter. Without magnesium at the level it should be, it is difficult to keep calcium and alkalinity at a steady and proper level. Natural seawater generally contains 1280 ppm of magnesium. Low magnesium can be one of the factors of nuisance algae growth, and most reef keepers agree that 1350 ppm works best in our little glass boxes. I prefer to dose magnesium to ~1350 ppm before adjusting calcium and alkalinity.
~ Mindy

SPS fanatic.

Last edited by Myka; 11-10-2011 at 12:09 AM.

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