Not all water is chemically equal. If you sent samples of tap water from New York City; Des Moines, Iowa; and Pasadena, California to a lab for analysis, the results would be very different. That's because water contains more than just hydrogen and oxygen molecules. It can also contain varying amounts of heavy metals, dissolved minerals, gases, nitrogen compounds, and chemicals such as chlorine-not to mention organic and inorganic pollutants.

 If you've been keeping fish long enough to consider breeding them, you're already aware of the very important role that good water quality plays in the health and well being of your fish. But it is worth looking at the impact some properties of water can have on the breeding process, because fish that usually aren't fussy about the water you keep them in can get downright finicky when it comes to spawning. In addition, the presence of certain compounds in the water can be damaging to the eggs or fry.

 Here are some of the properties of water that can affect your success as a breeder:

 Nitrogen compounds- When organic materials such as fish waste and uneaten food break down in aquarium water, they produce nitrogen-based compounds such as ammonia, nitrites, and nitrates. If you've been keeping fish for even a short time, you probably already know that the presence of even small amounts of ammonia and nitrites in the water is stressful to them. What you may not know is that eggs and fry are even more sensitive than adult fish and may be stunted or die if levels are even a little bit elevated. It's important to make sure there are no traces of ammonia or nitrites in breeding or rearing tanks, and to make sure nitrates stay within acceptable limits. The best way to reduce nitrates is to do a water change. The best way to prevent ammonia and nitrites is to use a mature filter in the tanks, do regular water changes, and test the water frequently.

 pH- This is a measure of the acidity or alkalinity of water. The pH scale runs from 0 to 14, with 7.0 being considered neutral. Water with a higher pH is referred to as basic or alkaline, and water with a lower pH is referred to as acidic. Each fish species has a preferred pH range within which it is most comfortable, although many can adapt to a higher or lower pH.

 But just because they can live in it doesn't mean they'll breed in it. Because of the way fish have evolved in their native habitats, the pH at which they'll spawn is often somewhat lower than their "everyday" pH, and they're less likely to be flexible about it. In addition, pH can affect the male-to-female ratio of some species of fry; for instance, rainbowfish that breed in acidic water often produce more males, while alkaline water produces more females.

 If you wish to breed fish that prefer a –pH lower than that of your tap water, you can alter it using a variety of methods: filtering the water overpeat (be sure to use aquarium-grade peat, since the peat available in garden centers may contain harmful fertilizers); bubbling carbon dioxide into the water, or using commercial compounds, such as humic acid extracts, that are available at your aquarium shop.

 Fish rarely require a pH higher than their normal range in which to breed. However, if you wish to keep fish that prefer water more alkaline than that of your tap, you can increase it by adding oyster shells or rocks containing limestone to the tank, or by adding baking soda.

 Hardness- This is a measure of the dissolved mineral salts, particularly calcium carbonate and magnesium carbonate, present in water. These salts, which occur naturally in the earth, dissolve when they are exposed to the water in underground springs. The quantity in the water depends on where it comes from and the treatment processes it undergoes. Water with a high proportion of dissolved minerals is called hard, and water with a low proportion is called soft.

 Test Kits

Most aquarium stores sell two types of water test kits: paper strips that change color according to the pH level or the presence of nitrogen compounds, and liquid reagents that are added drop by drop to vials of tank water; the water itself turns a color if ammonia, nitrites, or nitrates are present. Test strips are easier, but liquid reagents are a bit more accurate—something to keep in mind if you want to breed very sensitive fish.

 The hardness of water is critical to breeding because it affects osmotic pressure-the flow of water across cell membranes. While adult fish can usually adapt to water that is harder or softer than their optimal range, their eggs and sperm often cannot. If the water is too hard, the outer membrane of the fish's eggs may toughen, hindering fertilization or preventing the eggs from hatching. And if the water is too soft, the eggs and sperm may take on so much water that they literally burst.

 The hardness of the water also determines its buffering capacity-that is, its ability to absorb acids (such as those produced by fish waste) without a drop in pH. If the buffering capacity of your water is too low, your tank may experience pH swings that could harm your fish, eggs, and fry; adding baking soda will increase the buffering capacity, but will also raise pH. On the other hand, if your water has a high buffering capacity, you will have to go to considerable trouble to lower the pH, because the buffers will keep pushing it back up. To solve this, you can process your water through an ion exchanger that swaps magnesium carbonate and calcium carbonate for sodium (a salt that has no effect on hardness), or through a reverse osmosis unit, which removes all dissolved solids from the water by filtering it through a membrane.

 There are many different ways to measure and express hardness. One common scale measures it in terms of dH, or degree of hardness, while another expresses it as GH, or general hardness, and a third measures it terms of ppm, or parts per million. All are equally valid-the one you use is likely to depend on which brand of test kit you purchase. Many also include conversion charts to help you switch from one to another.

 Dissolved gases- Water absorbs atmospheric oxygen and carbon dioxide in a process called gas exchange, which takes place at the water's surface. Fish take in oxygen as water passes across their gill membranes, and give off carbon dioxide in return. The more agitation at the water's surface, the greater the amount of oxygen that passes into the water, and the greater the amount of carbon dioxide released. This is particularly critical in incubators and rearing tanks, since eggs and fry are even more sensitive to low oxygen levels than adult fish. For example, eggs will grow fungus if the oxygen level of the water is too low. Aerating the tanks with an airstone is a good way to facilitate gas exchange.

 Temperature- While it's perhaps unfair to call temperature a property of water, it does play an important role not just in inducing spawning but also in the rate at which fry hatch. Eggs that incubate in warmer water will hatch sooner than those kept in cooler water; likewise, higher temperatures encourage fry to grow faster.

 Conductivity- Conductivity is a measure of the water's ability to carry an electrical current. It is related to hardness, in that water with many dissolved minerals is more conductive than extremely soft water. However, conductivity, which is measured in units known as microsiemens, also measures sodium ions, as well as some other substances that do not affect the hardness of water.

 Although amateur breeders traditionally have not paid much attention to conductivity, some scientists believe it may have a larger role in spawning some species than initially thought. For instance, researchers have been able to prompt some fish in the Mormyridae family to breed simply by changing the conductivity of the water in which they were kept. And some species, such as discus, also breed more readily when the conductivity falls within a particular range. Most species that are fussy about conductivityis something to keep in mind if you're interested in trying your hand at some of the hard-to-breed fish. Electronic conductivity meters can be ordered through companies that manufacture aquatic instruments.

 Heavy metals- Tap water can also hold dissolved-metal ions such as iron, magnesium, lead, zinc, and copper. This last in particular is poisonous to fish and sometimes leaches into tap water from old copper pipes. It should go without saying that if heavy metals are present to such a degree that fish are stressed, they are unlikely to breed. Most water conditioners contain chelating agents that bind heavy metals in tap water. Alternatively, they can also be filtered out by an ion-exchange water softener.

 Chlorine and/or chloramines- These chemicals are used by utility companies to purify the drinking water supply. They are toxic to fish, and must be neutralized either by aging the water for 24 hours before adding it to the tank or by using a water conditioner. Although any experienced fishkeeper already knows this, it's worth repeating, especially since both breeding fish and raising fry often require frequent, large water changes.