Philip Shaddock: Note that since I have been using crushed coral and two sponge filters in my tanks, plus feeding my fish fresh high quality food sparingly, I have not had to think much about water chemistry. But a good background knowledge of water chemistry is useful to the guppy breeder when it comes time to diagnose guppy health problems.

Ammonia, Nitrite, and Nitrate

Ammonia (NH3) is a soluble gas produced by the metabolic processes of guppies and the break down of other organic matter, including uneaten food. Overfeeding your fish can cause a rapid spike in ammonia levels, beyond the 0.2 mg per liter tolerable to guppies. This is why is it is important not to feed sick guppies, guppies that are new to your fish room or overfeed guppies placed in a newly set up aquarium.

Established aquariums are teeming with bacteria that quickly convert ammonia to nitrite (NO2), which is slightly less toxic and can be tolerated by the fish up until 0.5 mg/liter. Nitrite is itself converted to nitrate (NO3) in established aquariums, which can reach a concentration of 100 mg/liter before it too becomes lethal. At this point you’ve reached the end of the nitrogen cycle, and must physically remove nitrate by changing a portion of the water (30%) once a week, depending on the stocking level of the aquarium.

The article on the nitrogen cycle goes into this topic in more detail. There are test kits that will measure levels of the three nitrogen cycle chemicals. If your fish are gasping near the surface, nitrite poisoning (rather than oxygen starvation) may be the cause. Ammonia impairs the fish’s sensitive gills, preventing them from acquiring oxygen. A high nitrite level means that ammonia is not being converted to nitrate by denitrifying bacteria. Your biological cycle is therefore not working.

The harmful effects of ammonia are affected by the aquarium’s pH level. Alkaline water (where the pH is above 7.0) sharply amplifies its toxicity.

Buffering Capacity (Carbonate Hardness or KH and Alkalinity)

Most aquarists intuitively understand hardness as the amount of dissolved minerals in the water and pH as the acidity or alkalinity of the aquarium water. Buffering capacity (a.k.a. carbonate hardness or KH or alkalinity) not only suffers from a profusion of names, it does not call a nice simple image to mind, making it difficult to understand.

Let’s put the image of a sponge in your mind. Like the capacity of a sponge to hold water, buffering capacity acts like a sponge soaking up the acids that cause pH to drop. (The more acidic the water, the lower the pH.) Acids may be present in your source water. However, over time the biggest source of acids in the tank is fish metabolic processes. Now let’s get more technical. Buffering capacity is the measure of carbonate (CO3–) and bicarbonate (HCO3-) ions dissolved in water. They are present in natural water systems, in municipal water supplies, and bottled water. The level of carbonate hardness in your tap water is dependent on the source of the water and the treatment processes it goes through.

KH doesn’t affect fish directly, so guppies do not have to be matched to a particular KH range. However KH has an indirect affect on the health of your fish.

When the aquarium has some carbonate buffering in it, the bicarbonate ions will combine with the excess hydrogen ions to form carbonic acid (H2CO3), which then slowly breaks down into CO2 and water. Since the excess hydrogen ions are used in the reaction, the pH does not change very much. Over time, as the carbonate ions are used up, the buffering capacity will drop and larger pH changes will be noted. From this it is clear why aquariums with low KH seem unstable – as acid is produced by biological action, the KH is used up; when it is gone, the pH is free to drop rapidly as H+ ions are generated. Acids are introduced to the aquarium as a result of the metabolic processes of fish, through acidic liquids and foods, tap water with high or low pH values, and other sources. In the case of such acids as nitric acid (nitrate) tanks with a relatively high concentration of carbonates and bicarbonates can absorb and neutralize the added acid. However, like a sponge, the ability of the aquarium to soak up or “buffer” acids reaches a limit. As acids continue to be added, the pH changes rapidly.

Driven by the fish’s metabolic processes, the nitrogen cycle’s end product is nitric acid, which dissociates into free hydrogen ions (H+) and nitrate. Without the presence of carbonates and bicarbonates to absorb the free hydrogen ions, the tank’s pH drops, adversely affecting the fish.

Buffering capacity test kits measure carbonate and bicarbonate concentrations in water. In North America, the amount of minerals dissolved in water is commonly measured in parts per million (ppm) of calcium carbonate (CaCO3). Even though the test results are expressed in terms of CaCO3, what is meant is that the reading is equivalent to that concentration of CaCO3. The Europeans have their own way of measuring hardness, called German degrees of hardness or odGH (sometimes abbreviated to degrees of hardness, odH or dH). One German degree (1 odGH) is 10 mg of calcium oxide (CaO) per liter. You can translate among the measuring systems. One ppm is roughly equivalent to 1 mg per litre. One German degree is 17.9 ppm CaCO3.

KH values between 50 ppm and 200 ppm are optimal. Water below 50 ppm KH is subject to wide fluctuations in pH values, harmful for guppies, which do not react well to unstable tank conditions. Values over 200 ppm result in water with very high pH values, harmful for guppies. A tank with low KH readings (below 100 ppm) should be monitored regularly, especially if you do not do regular water changes. The nitrogen cycle drives the established tank’s pH down over time. In new tanks, the pH values can bounce widely as the tank goes through chemical changes. If the pH drops more than two tenths of a degree (35 ppm) over a month, consider increasing the carbonate hardness or perform partial water changes more frequently.

The Word Maze The terms carbonate hardness (KH), alkalinity and buffering capacity are often used to describe the same thing. However, buffering capacity refers to the ability of carbonates and bicarbonates to absorb pH changes. The term “alkalinity’” should not be confused with the term “alkaline.” Alkalinity refers to the acid binding capacity of water, while alkaline refers to a solution that is a base, with a pH less than 7. Although technically not identical in meaning, for practical purposes the terms can be used interchangeably.

Adding Distilled Water to the Aquarium

Purified or distilled water has virtually no carbonates or bicarbonates in it. This means it has no buffering capacity. Adding even a little bit of acid to the water will cause a sudden change in the water’s pH values, stressing or shocking the fish. Distilled water must be conditioned by adding calcium, magnesium, carbonates and bicarbonates to it.

General Hardness (GH)

Water hardness is of interest to aquarists for two reasons: to provide the proper environment for the fish and to help stabilize the pH in the aquarium. There are two types of water hardness: general hardness (GH) and carbonate hardness (KH). A third term commonly used is total hardness, which is a combination of GH and KH. Since it is important to know both the GH and KH, the use of total hardness can be misleading and should be avoided.

When you hear that a species of fish prefers hard or soft water, it is general (GH) that is being referred to, not carbonate hardness (KH). General hardness (GH) is the measure of the concentration of metallic ions in water, particularly magnesium (Mg++) and calcium (Ca++). While other ions can affect GH, their effect is negligible or difficult to measure. By far, the most abundant ion present in water is calcium. General or total hardness refers to the total content of all these dissolved minerals.

The visual evidence for hard water is a white lime that is deposited at the water line in kettles or on the inner surface of pipes.

Rainwater becomes hard when it passes through soil and over rock, dissolving such minerals as calcite (calcium carbonate) and dolomite (calcium carbonate and magnesium carbonate). Calcium and magnesium ions are present in municipal, well and bottled water. The concentration levels depend on the source of the water and the treatment process it has undergone.

GH measurements standards are similar to those described in the Buffering Capacity section. In North America, the amount of minerals dissolved in water is measured in ppm. The Europeans measure German degrees. Convert German degrees to parts per million (ppm) using the 17.9 factor (1 degree German equals 17.9 ppm). Here is a table that shows what people mean when they describe the hardness or softness of their water.

How hard is hard? This table lays it out.

Hardness Table

German Degrees	ppm	Description
0 – 4 odGH	0 – 70 ppm	very soft
4 – 8 odGH	70 – 140 ppm	soft
8 – 12 odGH	140 – 210 ppm	moderately hard
12 – 18 odGH	210 – 320 ppm	hard
18 – 30 odGH	320 – 530 ppm	very hard

pH

Water can be an acid, a base, or neutral. Its acidity or alkalinity is measured in terms of pH. A pH of 7 is said to be neutral, a pH below 7 is acidic and a pH above 7 is alkaline. The pH scale is reverse logarithmic. A movement of 1 unit on the pH scale represents a 10-fold change in pH. To avoid stressing your fish, they should not be subjected to a change greater than .2 units.

Guppies can comfortably live and breed in a range of values from 6.8 to 7.6, and can be adapted to water pH 8 or even higher. They react poorly to sudden changes in pH. If guppies are moved from one tank to another, it’s better to put it into a tank with slightly higher pH than slightly lower pH.

The pH is affected by the water hardness, the type of gravel used if any (dolomite raises pH), and the presence of organic material like fish waste or decaying plants. There can be fluctuations over a 24 hour period, so pH is best measured at the same time each day.

As the GH of water rises, its pH rises. You can also raise the pH of water by raising its carbonate hardness.

The presence of CO2 (carbon dioxide) also affects the pH of water. As the level of dissolved CO2 rises, the pH drops. CO2 is a byproduct of nitrification and fish or plant respiration. As the CO2 is produced, carbonic acid and bicarbonate are formed, reducing the pH.

CO2 is produced by fish during the nitrogen cycle, and it is constantly released into the air at the surface. In an aquarium that is not overloaded with fish, dissolved CO2 levels are in the range of 2-3 ppm. Regulating the pH in the aquarium with a CO2 system is really only useful for planted aquarium. In guppy tanks you want to maximize oxygen and minimize CO2. Alkaline buffer compounds are used to raise the pH value.