Alkalinity, the acid-neutralizing capacity of water

Alkalinity ("buffering capacity") measures any of the substances dissolved in water that can take up or release the H+ ions that affect pH. They form a kind of reservoir or temporary "sink." Carbonates and bicarbonates of calcium or magnesium are the ordinary buffers, but phosphates function too. In aquarium circles we still measure alkalinity as if it were entirely due to "degrees of carbonate hardness" or "KH"— from the German Karbonathärte "carbonate hardness." One degree KH is equivalent to 17.8 mg/L CaCO₃.
 
Alkalinity is a measure of the acid-neutralizing capacity of a water. Acid-neutralizing capacity means the ability to accept acid without a subsequent drop in pH. Alkalinity is basically a measure of how much antacid is dissolved in the water. The more acid that can be added to a water before the pH starts to drop, the higher the alkalinity. Normally, adding acid to a solution would cause a drop in the pH, since this increases the number of hydrogen ions. But if a hydrogen ion can be neutralized by a base, then it will not contribute to the pH, so the pH will not change. A water that can accept acid without a subsequent drop in pH is said to be buffered. Therefore, alkalinity is the amount of buffering in a water. 
 
Alkalinity represents part of the total hardness, which is called "GH," from the German "Gesamthärte" "total hardness". George Slusarczuk gave the members of Aquatic-Plants Digest a capsule history of the idea of "hardness" in water, which is archived at The Krib. 
 
A "buffer," then, is a dissolved compound combining an acid and a salt (that's mostly a carbonate or bicarbonate in our case, though it can also be a phosphate), which tends to maintain water at a constant pH, by accepting or releasing H+ ions in response to small changes in H+ concentration. Common buffers in living organisms include bicarbonate (HCO₃) and phosphates (H₂PO₄ and HPO₄) in the blood. Blood is buffered at a pH of about 7.7 by bicarbonates that release H+ ions to form carbonic acid: HCO₃ + H = H₂CO₃ the equation reads going in either direction, giving or receiving that H ion.
 
Phosphate buffers. Fishkeepers who are determined to buffer their pH at fixed levels near neutral still sometimes resort to phosphate-based buffers. Some unique situations may ensue: high levels of phosphate can interfere with plants' uptake of zinc and iron. Trying to compensate with additional iron fertilizer is generally ineffective: leaves will still yellow from micro-nutrient deficiencies. Thus phosphate-based buffers are dis-recommended for planted tanks.
 
Phosphate from buffers will give false high readings in KH tests, because they measure the phosphates right along with carbonates/bicarbonates. When high levels of calcium and magnesium are present, phosphate buffers can even cloud the water, as Ca and Mg precipitate onto the buffer. Residual negative charges continue to repel the complexes and keep them in circulation.
 
Though other soluble minerals besides carbonates/bicarbonates and phosphates form compounds commonly called "salts," no other ordinary salts, including rock salt ("table" salt or sodium chloride), have any measurable effect on alkalinity. (You can ignore borate, unless your condo is in Death Valley! Borate does figure in some "Rift Lake" buffering salts, though, and in some artificial seawater mixes. But not at levels that would affect the buffering.) A pinch of common "table" salt, in other words, doesn't add to your buffer, but a pinch of "marine" salt will include some calcium and magnesium carbonates, components that will add to your buffer.
 
Once again, other substances that don't figure in assessing the alkalinity (or "KH"): sulfur dioxide, nitrogen compounds, both organic and inorganic, chlorides, sulfates, silicon compounds, aluminum and copper and other heavy metal compounds, and the whole range of organic humic substances.
 
KH and plants. What plants "require" most is light and carbon derived from CO₂. In "hard" highly buffered water, the C is locked up in its carbonate form. When carbon dioxide is artificially diffused into water, it combines with carbonates, temporarily reducing the KH component. A little of the CO₂ also forms carbonic acid, lowering the pH. In soft water, such raised levels of CO₂ can bring pH to unacceptably low levels. In planted aquaria, if you are diffusing CO₂ even though your carbonate levels are low, you are often advised to add buffering. It seems rather elaborate to find yourself adding carbonate to compensate, when originally CO₂ was being added because the carbonates in the water had the carbon all locked up — but the resulting garden aquaria are very handsome all the same.
 
Calcium carbonate is useful for providing a buffer in very soft waters. A handy cheap source is crushed coral, or aragonite. Aragonite and calcite are two mineral forms of calcium carbonate (CaCO₃), differing only in their crystal structures, which affect irrelevant things like melting points and density. More to the point, aragonite is more easily soluble than calcite. Actually, these carbonate minerals aren't very soluble in water;  it's the carbon dioxide in its carbonic acid form that "dissolves" them.
 
Coral skeletons, and clam shells too, are built of calcium carbonate. So are the shells of freshwater snails. If carbonates were readily soluble, reef structures would melt away. Your crushed coral marine "sand" will include lots of tiny shells and shell fragments. Crushed coral/aragonite/calcite belongs in the filter, where you can control it, rather than mixed into the substrate— unless you're running a Rift Lake Tank of highly alkaline water. Don't hunt for an expensive fine-ground "instantly available" aragonite: you are looking for a gentle, long-term effect, not a "boost".
 
Aragonite. Aragonite is nothing more special than some crushed coral or even an eroding snail shell or two. I remember when the "crushed coral" substrate for marine tanks was generally available by the cupful at your LFS, sold cheaply from an open bag. Put a little crushed coral in a bag in the filter. The removable bag helps you control the amount of buffering you're getting, and the bag will limit the number of shell bits that get into your impellor.
 
In brief, your best "instant" boost to the pH would come from sodium bicarbonate ("bicarb" from the drugstore). It's easily soluble, and contributes all its carbonate to your alkalinity in a flash. The bounce it gives to the pH can stress the fish. For longterm stability, however, you want the aragonite from crushed coral. Since eggshells are also made of calcium carbonate, you could scrunch a couple of clean eggshells and add them to the filter.
 
How much crushed coral should you be using to increase the alkaline buffer and stabilize pH around neutral, 7.0? ArkyLady posted her experience at AquariaCentral, 17 Dec 2002:
 
"I thought I'd report back how my experience with using crushed coral to increase my KH levels worked out: Test results before adding anything: 12/9/02 - 55 gal: KH 0ppm, GH 50ppm, pH 6.2 10 gal: KH 0ppm, GH 50ppm, pH 6.2 Test results 8 days later with no water changes: 12/17/02 - 55 gal: KH 40ppm, GH 75ppm, pH 6.8 10 gal: KH 40ppm, GH 75ppm, pH 7.2 I added 2 TEAspoons to the 10 gal and 2 TABLEspoons to the 55 gal. I just put the crushed coral down into the toe end of a pair of kneehighs and dropped it down into the filter. I removed about half of the crushed coral from the 10 gal because I don't want it to go up any higher than it is now. I don't want the 55 gal to get any higher either because the fish in there prefer a lower pH, but I haven't changed the amount of crushed coral in that tank yet. From what I've read on this topic so far, a KH of 40ppm is still a bit on the low side. However, it's much better than the 0 I had before, so I'm happy with the readings."
 
So that should give you an idea.