The Skeptical Fishkeeper: Technical Definitions

Author: Laura Muha

GH is an abbreviation for Gesamthaerte. This is German for “total hardness.” GH is the sum of divalent cations in solution. In aquatic habitats, the vast majority of these ions are calcium and magnesium.

KH is an abbreviation for Karbonathaerte, German for “carbonate hardness.” This is a misnomer, since KH does not measure cations; it therefore does not measure any type of hardness. It measures anions that contribute to alkalinity. In natural aquatic habitats the majority of these are carbonate and bicarbonate.

While many adult fish seem able to adapt to water that is harder or softer than their optimal range, they may not breed successfully in it. Water that’s too hard may cause the outer membrane of a fish’s eggs to toughen, hindering fertilization or preventing the eggs from hatching. And if the water is too soft, so much of it may pass through the cell membranes of the eggs and sperm that they literally burst.


There are two letters of the alphabet that have always struck confusion, if not outright fear, in my hobbyist’s heart: “G” and “H.”

Taken individually they’re innocent enough. But put them together and they add up to what, to me, has always been one of the murkiest concepts in fishkeeping: “general hardness.”

Or is it “German hardness”? No, wait … maybe I’m forgetting a letter. Isn’t there a “D” in there somewhere? As in dGH? Or is it just DH? I’ve seen references to all three in aquarium literature, along with something called “KH”—although one of my test kits calls it “dKH.” What’s the difference between all of them? And what’s their significance to my fish?

The Mental Block Gets Lifted

My head hurts just thinking about it, and I might have avoided doing so indefinitely had I not decided last summer that caring for my ever-growing collection of tanks was taking up way too much of my time. The obvious solution would have been to find new homes for some of my fish. But with a logic that only another hobbyist could understand, I came up with a better idea: to buy yet another tank—a really big tank—so I could condense most of my smaller tanks into it.

“That way,” I enthused to my husband, “I’ll only have one or two freshwater tanks to take care of, instead of five or six!” Having heard similar rationalizations in the past, he looked a bit skeptical; somehow, the number of tanks in our house always goes up, never down. But he didn’t say no, so I set about figuring out whether the fish I wanted to keep in this new tank could handle the same water conditions. And that is what brought me to a moment of truth: I could no longer put off confronting my long-time mental block about GH, or whatever it is that it’s called.

I bring up that particular water parameter and not the others (temperature and pH), because those two didn’t differ significantly in the tanks I wanted to combine: a 55-gallon tank containing pearl gouramis, platys, and emerald catfish; a 55-gallon tank containing rainbowfish, rosy barbs, and zebra danios; and several 10-gallon tanks containing Neolamprologus multifasciatus, a tiny shell-dwelling cichlid from Lake Tanganyika. (I realize this is not the most common mix of fish for a community tank, but that discussion will have to wait ‘til next month!)

A Question of Water Parameters

In the meantime, let’s get back to the water parameters, most of which, as I’ve already noted, didn’t worry me. I keep the temperature in all my aquariums at 77°F, and the pH of the water in all of them except for the shell-dweller tanks was 8.1, which is exactly what it was when it came out of our tap. (As I noted in my April 2006 TFH column on pH, I’ve never adjusted it, and although conventional wisdom says that many of the species I keep prefer water that is neutral or acidic, mine seem to live happily and breed regularly in our high-pH well water.)

In the cichlid tanks, the addition of coral substrate and cichlid salts boosted the pH higher than that of the other tanks, to about 8.4. But multis are tough little fish and, according to aquarium literature, they can easily handle a pH range of 7.8 to 9, so I was confident they would adjust if the pH of their new home was a little lower than the pH to which they were accustomed.

Water Hardness

The hardness of the water was another matter, however, because when it came to that parameter, there was a significant difference between the cichlid and community tanks. According to my test kit, the water in the “regular” tanks was moderately hard, but in the cichlid tanks, it was “very hard.” Because the test involved comparing colors on a test strip to colors on a chart, and those colors never quite seem to match, I couldn’t figure out the exact numbers, but my best guess was that it was somewhere around 100 parts per million in the regular tanks, and twice that—200 ppm—in the cichlid tanks.

Calculating the Numbers

Since I sensed a column topic brewing, and I wanted to be able to do more than guess at the numbers, I purchased a second test kit hoping it would give me more precise results. This one involved dripping reagent into a test tube of aquarium water and counting the number of drops that it took to change the water from orange to green. The problem was that the results for that kit were given not in parts per million, but in degrees. In the “regular” tanks, it took 5 drops to elicit the change, which according to the instructions meant that the water had 5 degrees of hardness.

In the cichlid tank, it took 11 drops, meaning that it had 11 degrees of hardness. I wasn’t sure how those figures compared to the “ppm” results of the test strips, but the implications were clear anyway: If the occupants of my various tanks were to live together successfully, the cichlids were going to have to get used to softer water, or the community fish were going to have to get used to harder water. The question was whether they could.

Understanding Water Hardness

Before I tackled that issue, I decided it might help to have a definition of water hardness, and for that, I turned to one of my favorite (and easiest-to-reach) sources: My father, Dr. George Muha, a professor emeritus of chemistry at Rutgers University. “I told you that you should have taken more chemistry in school,” he said when I put my question to him. My poor father has never quite gotten over the fact that none of his three children followed him into the sciences, and his tone of voice suggested that he was rolling his eyes at the other end of the phone.

“You’re getting way too confused about this,” he said. Hardness is just a way of measuring how many metal ions—specifically calcium and magnesium—are dissolved in water, he explained; simply put, hard water has lots of them and soft water does not. The explanation for the difference can be traced to the journey water makes from its point of origin to our faucets. As it travels, it comes into contact with rocks, and as it does some of the minerals in these rocks dissolve into the water. Since the type of rock varies from one place to another, so does the mineral content of the water—and thus its hardness.

Parts Per Million

One way to measure hardness is in parts per million, a scale commonly used in the United States; the other is in degrees, a scale known as the German scale after its country of origin. Both are used regularly in the aquarium industry, which is where some of my confusion originated. For the record, one degree of hardness equals 17.9 ppm, but the math-phobic among us needn’t worry, since conversion charts are available online and with the instructions that come with many test kits. By consulting one, I quickly determined that the hardness of the water in my shellie tanks was 11 degrees or 196.9 ppm, and in the “regular” tanks, it was 5 degrees, or 89.5 ppm.


As for that alphabet soup of acronyms that I listed at the beginning of the column: GH most often seems to refer to the American scale and dGH to the German scale, although I’ve seen them used in reverse. I’ve also have seen DH—which depending on who you ask means either “degrees of hardness” or “Deutsche hardness”—used with both scales. But if you think about it, the acronym you choose or what it stands for isn’t as important as knowing whether you’re measuring parts per million or degrees. Describing the hardness of your water as 17, for instance, is useless, since water with a hardness of 17 degrees is radically different from water with a hardness of 17 ppm.


The one that acronym that does means something very different is KH, sometimes also referred to as dKH, from the German word meaning “carbonate hardness.” Carbonates are important because they buffer water against swings in pH by neutralizing acids in the water, including those produced during the breakdown of organic waste in fish tanks. It’s a subject I’ve written about before (TFH April 2006) so I won’t go into it here, except to say that many fishkeepers are moving away from using the term “KH,” since it is misleading, and instead referring to it as the alkalinity or buffering capacity of the water.

Water Hardness and Fish

But whatever you call water hardness, and however you measure it, the bottom line is what it means to your fish, which is something a lot different than what it means to you and me. For us, water that’s too hard or too soft is more of an inconvenience than anything else; hard water can cause limescale to build up on pipes (and aquarium fixtures) and make it difficult to work up a good lather with soap or shampoo; soft water creates too much lather and makes it difficult to rinse off.

But to fish, water that’s too hard or too soft is more than an annoyance—it can be a significant stressor. That’s because freshwater fish live in an environment less saline than their body fluids, and if you remember anything about osmosis from science class you know that when you have solutions of two different concentrations on either side of a permeable membrane—such as the cell walls of a fish—they have a tendency to want to equalize. That means that in the case of a freshwater fish, water from the environment is constantly trying to flow across its cell membranes and into its body. If this process were unchecked, not only would a fish’s cells explode like overstretched water balloons, but its internal saline balance would be thrown completely (and potentially fatally) out of whack. So the fish must work hard to control the amount of water that moves in and out of its cells through a complex process known as osmoregulation.

All the literature I could find on multis suggested that they needed very hard water, so I decided to set up an experimental tank without the cichlid salts and coral sand so that the hardness would be the same as it was in my regular tanks, and to transfer a few of my ever-growing colony into it to see what would happen. I had my answer a few weeks later when I peered into the tank a month or so later and saw fry darting among the shells.

Clearly, water hardness was not much of an issue for multis; the only question that remained was whether they would get along with the other fish I proposed mixing with them.

But fish compatibility, as I said, is a subject for next month.