Salty Q&A: Calcium and Alkalinity Out of Whack

Maintaining stable parameters in the accepted range is much better than chasing "ideal" numbers

Maintaining stable parameters in the accepted range is much better than chasing “ideal” numbers


I’m having an issue with the calcium and alkalinity levels in my reef tank. Right now, the calcium is at 380ppm, which is at the low end of the acceptable range, and the alkalinity is at 10dKH, closer to the high end of the acceptable range. I’d like to get the calcium up over 400ppm so I have a greater margin for error. But whenever I add more calcium supplement than usual, I notice that the alkalinity level drops afterward. Any idea why this is happening? Is it coincidence or cause-and-effect?” – Submitted by Ross C.


Thanks for your question, Ross! The drop in your alkalinity level subsequent to increased calcium supplementation is likely cause-and-effect rather than coincidence, and the reason for this is fairly easy to understand—even for a right-brained hobbyist like me.

You see, any given volume of water can hold only so much in the way of dissolved solids. If your aquarium water is already holding all the calcium and buffering compounds it can, adding more of one will tend to displace some of the other. Thus you can get a seesaw relationship between the two levels if you don’t dose them in a balanced fashion.

Given my characteristically low-tech thought process (i.e. simple mind), the best explanation for this phenomenon that I’ve come across is Anthony Calfo’s Marble Analogy, which I first read in his Book of Coral Propagation (it may appear in various places online as well, but this book is a worthwhile read in any case). I’m fairly certain I’ve quoted Calfo’s analogy in a prior post or two, but it bears repeating for anyone trying to understand the calcium/alkalinity interrelationship, so here goes:

“Imagine a bowl that holds one hundred marbles representing the total dissolved solids in seawater in a given system. If red marbles represent calcium, and blue marbles represent alkalinity, the bowl can still only hold one hundred marbles no matter what mix of colors they are. Thus, if seventy red marbles are the equivalent of 400ppm calcium and the remaining marbles are blue, the only way to increase calcium is to displace alkalinity (remove blue marbles).”

Now, with that said, my advice would be to maintain your calcium and alkalinity levels right where they are. 380ppm may be at the lower end of the desired calcium range, but it’s a perfectly acceptable value, and at 10dKH, your alkalinity is good to go as well. Remember, it’s generally better to maintain stable water parameters, even if they’re at the low end of the desired range or even slightly outside it, than to have those levels bounce all over the place as you chase after “ideal” numbers.

Photo credit: Kien Tran

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About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.


  1. Don’t forget that Magnesium also plays a role in Ca/Alka balance. Hence it’s sort of a misnomer being called 2 Part dosing since there is a 3rd part. Another great chemistry resource are the articles by Randy Holmes-Farley. There’s a range for proper MG so try to maintain that too.

    • Ha, you beat me to it Ellery. I was just going to mention about magnesium as well. It is vital, but I don’t quite know how best to insert it into the anaolgy without adding confusion. Jeff?


    • Jeff Kurtz says:

      Ah, yes! Thanks for raising the magnesium issue, Ellery and Jeff. Since you mentioned Randy Holmes-Farley, here’s a link to one of his articles in Reefkeeping that is well worth bookmarking: In it, he touches on a wide range of parameters, including magnesium, in much greater depth than I ever could.

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