Like the air you breathe, the water in your fish tank must mimic the natural environment your fish are used to as closely as possible. But what exactly do fish need in the water to keep them healthy? And what must stay out of their water not only for them to survive, but thrive? Here are the factors that impact water quality and what you can do to control and maintain the best water for your fish.

Water Properties

Water has myriad properties that need to be balanced in order to support life. The water qualities needed for a freshwater environment are different - and easier to control - than those needed for a saltwater tank. That's why beginners usually opt to start with a freshwater tank, which is what the following information pertains to.

First, what are the different ways you'll be able to control and maintain the ideal water quality, thereby promoting your fish's health and happiness, in your freshwater tank?

1. Water quality is monitored and measured in a number of ways:

• Salinity: how much salt is in the water. For the purely freshwater tank, this will be none.
• pH level: degree of acidity or alkalinity of the water.
• Hardness or softness: the amount of dissolved calcium and magnesium in the water.

• Chlorine or chloramine-antibacterial agents added to the water by public water authorities.
• Heavy metals such as dissolved copper, iron, lead or zinc can be present in public or well-water supplies.
• Ammonia and nitrogen by-products, which are the result of fish waste and uneaten food in the water.

• Dissolved oxygen, for the fish to breathe comfortably.
• Heat, to maintain an even, ideal temperature for your fish.

However, you can't mix fish that won't tolerate the same type of water; for instance, freshwater fish do not mix with those from the ocean. But even among freshwater fish, conditions aren't always compatible. For example, African cichlids require harder water (with a higher dissolved-mineral content), than the majority of tank fish, and goldfish cannot tolerate the warmer water (75 degrees F) most other tropical fish need.

Water Characteristics to Monitor and Adjust

The following three main properties of water quality are easy to monitor and adjust with commercial test kits:

1. Salinity

• Salinity is the measure of the amount of salt in the water. Salinity has many impacts upon the different types of fish. Freshwater fish absorb water through their skin, and excrete large amounts of water to prevent "drowning" due to large amounts of salt in their systems. In contrast, saltwater fish "drink" their water, and lose the water through their skin. Ocean fish are in constant danger of dehydration if the water surrounding them contains high amounts of salt, which robs the water from their bodies. Therefore, maintaining the proper amount of salinity in the water is vitally important to the type of fish you choose.
• Salinity is calculated by measuring the specific gravity, or density, of the water. A hydrometer is the device used to measure the SG of water. There are two types of hydrometers: floating and swing needle. The swing needle type is the preferred method; the floating hydrometer requires you to calibrate its readings based on temperature. The swing needle will provide accurate SG readings for the temperature required for your type of freshwater tropical fish.
• Like other aspects of water quality, SG must be checked regularly for variations. It is recommended the SG be examined every two or three days, because the fluctuations in the water's SG are usually caused by evaporation. In an aquarium, the salt remains in the water, but the water evaporates, so the SG goes up. New water must be added in order to maintain a constant level. A specific gravity rise of greater than one point causes excessive stress on the fish.
• Fish live naturally in freshwater lakes, rivers and streams will tolerate a very, very small amount of salt in their water. Some fish, such as the Archerfish, occur naturally in water nearer to the sea and live all their lives in slightly salty, or brackish, water. If you're trying to keep freshwater fish, don't mix them with fish that live in brackish or saltwater [See Marine Aquariums].

• pH is the measure of the acidity or alkalinity of the water. You'll need to check the water's pH level when you first set up your tank. With a testing kit, you take a small sample of water, add a few drops of special dye to it, and check the color of the water against a paper test strip. The color of the water will correspond to its level of acidity or alkalinity on the test strip.
• Most freshwater fish require a pH level in the neutral range, between 6.5 and 7.5, which is the range in most tap water. Seawater is characterized by its high pH, usually 8.0 to 8.3. The aquarium must replicate this neutrality or alkalinity.
• Once you know the water is within the proper range for your fish, you should test every two weeks to make sure the pH level is not fluctuating. Whatever the necessary level, it is important the pH remain constant. The pH level could be affected by several factors, such as overstocking or overfeeding your fish [See Maintaining Water Quality].
• If you need to adjust the pH of your water, buy a kit that includes chemicals to increase the pH (make it more alkaline) by adding sodium bicarbonate, or decrease the pH (make it more acidic) by adding phosphoric acid. You will rarely need to adjust the pH, such as if you're trying to encourage breeding in a species of fish and want to create optimum conditions that include a specific pH level.

• The hardness of water is defined by the amount of dissolved mineral salts in the water. It is measured by the amount of degrees hardness, or dH; each degree represents 30 mg of calcium or magnesium carbonate in the water. The amount of carbonate hardness is very important, since it affects the pH and the amount of carbon dioxide in the water.
• A majority of aquarium fish live in medium-hard or hard water. Each species has its own tolerances; it's recommended you consult a book or other resource to determine the correct dH for your fish. Usually, soft water has an acidic reaction (ph below 7), while harder water is more alkaline (pH above 7).
• Liquid reagents are used to measure the pH. If, after testing, you find your tap water is extremely soft, you may need to add minerals to the water. You can do this by adding a small amount of dolomitic limestone, coral gravel, or oyster shell "buffers." You would only need about 3 ounces for every 10 gallons of water, adding the buffer to the filter. If your tap water is too hard, you can add aquarium peat to the water, with the disadvantage that this will temporarily turn the water brown. When adding buffers or water softeners, you will need to monitor the pH level to make sure it's not fluctuating too much.
• Most aquarium fish that lay eggs prefer soft water; that is, water with fewer dissolved minerals in it. Fish that bear live young, such as mollies or guppies, and some African cichlids, require hard water.

You need to neutralize, minimize or eliminate these substances from your aquarium water: