Marine Aquarium Antacid: Understanding Alkalinity

Popular alkalinity test kit

Popular alkalinity test kit

When I first made the switch from freshwater to marine fishkeeping, I was somewhat befuddled by the term “alkalinity” as it’s typically used on the saltwater side of the hobby. During all my years of keeping freshwater systems, I had always used the term “alkaline” interchangeably with “basic.” In other words, with respect to the pH scale, I would describe any value below 7 (neutral on the scale) as being more acidic and any value above 7 as being more alkaline.

Related but different

What I soon discovered is that alkalinity is indeed related to pH—just not in the sense that I originally thought. In fact, your aquarium water can actually have a relatively high pH yet still be low in alkalinity. In this scenario, the pH is unstable and can plummet rapidly if an acid is introduced. So, clearly, the terms “alkaline” and “basic” are not synonymous.

Simply put, the alkalinity level (also called “buffering capacity”) of aquarium water refers to its ability to resist a downward shift in pH in the presence of an acid. I like to think of alkalinity as antacid for a marine aquarium (a visual that always resonates with me given my propensity for overindulgence at mealtimes). In aquarium systems, acids are introduced in the form of decomposing organic matter, the metabolic wastes of marine organisms, excess dissolved carbon dioxide, etc.

What’s buffering the water?

When it comes to buffering capacity in marine aquariums, bicarbonate and carbonate are the biggest contributors (with bicarbonate pulling the heaviest load and carbonate coming in a distant second). Thus, you’ll often hear/read alkalinity described as “carbonate hardness” and it’s these buffers that are measured by the alkalinity test kits available in the hobby. The target range for alkalinity is between 8 and 12 dKH.

How is alkalinity supplemented?

All the necessary buffering compounds are present in appropriate proportions in any quality synthetic sea salt mix, so performing regular water changes is usually adequate to maintain the proper alkalinity level in most fish-only marine aquariums.

However, in reef systems with a high calcium/alkalinity demand, such as those containing stony corals and/or giant clams, which make their skeletons/shells out of calcium carbonate, alkalinity typically needs to be supplemented beyond what’s provided through partial water changes.

This is usually achieved hand-in-hand with calcium supplementation via methods such as dosing balanced two-part liquid calcium/alkalinity additives, dripping kalkwasser (a.k.a. limewater) to replace evaporated fresh water, or installing a calcium reactor in the system.

A delicate balance

When supplementing calcium/alkalinity, keep in mind that the level of one has a direct influence on the level of the other. Water can hold only so many dissolved solids, so adding too much buffer will tend to cause the calcium level to drop and vice versa—sort of like a seesaw.


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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. pH neutral is seven, not zero.

  2. In a fowlr system ( for instance in a 2x2x1(H) tank with 4×3=12″ of total fish length, 10kg liverocks, 1″ aragonite sand bed ) is not the substrate capable enough to keep the pH & alkalinity in check ?

    • Jeff Kurtz says

      That coupled with regular partial water changes should be adequate to maintain alkalinity in a FOWLR system. Thanks for your question, Aayaan!

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