How to maintain water quality when farming fish

With an an adequate supply of clean water you can raise fish for your own consumption or for the market in the same way that you farm other meat-producing animals. As these tips will show, your giant aquarium is relatively simple and economical to manage.

Fish farming, long a major source of protein in China and in other parts of Asia, is developing rapidly as a rural industry worldwide. Fish produce more protein per hectare than other kinds of livestock, they are more efficient at converting feed into usable meat, and some even yield a higher proportion of meat than more conventional animals.

Fish are grown commercially in large ponds or dams, but on a small property a watering dam for stock can produce a significant proportion of a family's protein needs. On very small properties, an above-ground wading pool, four metres (13 feet) across and 60 centimetres (two feet) deep, can be big enough to raise worthwhile numbers of some species of fish and crayfish in one growing season of five to six months.

The types of fish you can raise as well as the number you harvest will depend on three main factors: the oxygen content of the water, the water quality and the water temperature.

Fish must have oxygen to survive. When oxygen dissolved in the water falls too low, the fish suffocate and die. Oxygen enters water by diffusion from the air and by the action of algae and other water plants. The fish farmer can get around natural deficiencies, such as the tendency of warm water to hold less oxygen than cold water, by using an aerator to supply needed oxygen. In fact, constant aeration is the easiest way to increase the fish harvest, since fish will grow faster when they have plentiful oxygen; the water also tends to be cleaner as bacteria can break down fish waste more rapidly in well-oxygenated water.

Water quality

The pH (degree of acidity or alkalinity) and the presence of organic waste play a large part in determining water quality. Fish will not thrive in water that has a pH much lower than 6 (acid) or much higher than 8 (alkaline). The ideal pH is 6.5 to 7.5.

Check the pH regularly:

• add agricultural lime to reduce acidity or add gypsum to reduce alkalinity.
• If your water supply is chlorinated, let it stand for two or three days to allow chlorine to break down.
• The waste which fish excrete is highly nitrogenous and must not be allowed to accumulate as it will eventually poison the water. If you are raising fish in a pool, you will need a recirculating filter to overcome this problem.
• An adequate filter can be made at home from a clean 210 litre (55 gallon) drum filled with either gravel and shell-grit or special plastic ringlets.
• Naturally occurring bacteria that grow on the filter medium convert the fish wastes to harmless substances.
• A small electric pump circulates water to the filter; gravity feeds it back to the tank.

Cold-water fish such as trout do best in water below 15° C (60°F), as long as the water does not freeze. They will not thrive if the water temperature climbs above 20° C (68°C). Warm-water fish such as silver perch prefer water temperatures above 20° C (60°F), and they do not grow well if the water drops below 15° C (60°C), although they will survive in temperatures as low as 5° C (40°C). For best results, match your fish to the climate and then let nature work for you.