Guppy breeders can get water from the municipal utility, from a well, or from a store. Or they can “make” their own water. Let’s deal with the last, first. Making water consists of removing impurities from a water source through reverse osmosis (RO) or some other means.
You might think that the best way to create optimal water conditions for your aquariums is to start with purified (RO) water and add chemicals as needed.
“Pure” water is not a very good goal for your water program. That’s because freshwater in nature is never pure and fish have evolved to live in water with a lot of minerals in it. The water that falls from the heavens dissolves gases and collects dust on its way down. In industrial areas rainwater often contains pollutants that will find its way into your fish. As it sinks into the ground or rushes down streams and rivers, it gathers up more dissolved matter. The chemical composition of water in rivers and lakes depends on local geological and climatic conditions. In places like Los Angeles, the water is so laden with salts as to be virtually liquid rock. In places like the Pacific Northwest, where it rains a lot, the water has so few minerals in it as to barely be measurable with a hardness (GH) test kit.
Fish (and plants) have evolved over millions of years to the specific water conditions in their native habitats. That’s why species like the Cichlid require very soft water, while Lake Malawi African cichlids require water that is very hard.
Fortunately for guppy hobbyists, guppies are found in a wide range of habitats, making them more adaptable than the average fish. One condition guppies will not adapt to very well is pure water. Pure water does not contain many of the trace elements required for its metabolism.
While we know of at least one top breeder who is forced by the adverse conditions of local water to buy water from a fish store and cart it to his fish room, most breeders rely on local tap water as a source for water in their aquariums. However, the water can rarely be used directly from the tap. Local water companies add chemicals to the water to make it safe to drink, commonly using chlorine or chloramines to kill bacteria. In recent years, concern about corrosive effect of acidic water on lead and copper pipes has meant some water utilities add pH-raising chemicals to the water. Your local water company might have a web site listing the chemical properties of drinking water. Alternately, contact them by phone or mail. They will probably send you a chemical analysis of the water.
You can also test the water yourself. It is likely that tap water must be conditioned before it can put in the tanks.
Testing local water must be done constantly. Water parameters may vary over time, especially season to season. During water shortages, some water districts acquire water from neighboring water districts. The neighboring district may have water with different water parameters. During summer months, bacterial levels usually rise. Many utilities raise chlorine levels to combat this. In the rainy fall, reservoirs fill with softer water than the dryer months. For these reasons, and more, testing your water on a regular basis is a good idea.
But like a lot of good ideas, it is probably not practical. Much more practical is to set up your own water reservoir in garbage cans or a large volume aquarium. If your local utility adds chloramines to the water, treat the water. Otherwise save the expense of chemicals and just aerate the water to at least overnight before using it. If the water is in the same room as your tanks and your tanks are heated by the air in the room, then you will have a reservoir of water at hand with the same parameters as your tanks.
Your water supply may come from a well. Well water is usually not treated by the local water utility, so you need not worry about such chemicals as chlorine or chloramines. However, ground water is sometimes contaminated by chemicals from an adjoining farm or a more distant factory, so well water actually demands sustained vigilance from the guppy breeder.
Well water is often hard water. It may contain high levels of dissolved gas, making it important that you aerate for several hours before testing it or adding it to your tanks. It may be supersaturated with CO2, which lowers the water’s pH, until it evaporates and the pH rises again. This is stressful for fish. It’s a good idea to have the water analyzed professionally. This can cost between US $20 and $100.
If your water comes out of the tap at about 7.5 pH, then you are lucky. If it is 7.0 pH or below, you are going to have to adjust its pH up. You can do this with chemicals from the LFS, but it is much easier to add crushed coral to your water reservoir and tanks (in the filter or a handful on the bottom). The crushed coral buffers the water to about 7.8 pH and maintains it there. It also acts as a substrate for nitrifying bacteria. Adjust your guppies slowly to any change in pH.