In order to succeed at keeping fish, ensure that you are providing the best water conditions possible. Too often, disease and death among fish is a result of bad water quality. The following are water quality parameters that you must adhere to in order to be successful.
Chlorine and chloramines are commonly used to disinfect public water supplies. Both are lethal to fish. Eliminate with water conditioners, designed to neutralize poisonous compounds present in tap water.
Temperature should be between 65 and 85 degrees. Goldfish and koi will thrive below 65 degrees, but disease treatment is much more effective at higher temperatures, where the fish immune system is functioning better. Be sure to keep your holding facilities in the shade if possible to avoid high temperatures.
Fish need oxygen in the water to survive. A lightly stocked pond may not need additional aeration. However, if you have more than just a few fish in your pond or holding tank, additional aeration may be needed. If the water is low of dissolved oxygen, the fish will come to the top and stick their noses out of the water, seeming to gasp for air.
pH is the measurement of hydrogen ions in your water. It is vitally important to continually monitor your pH level, attempting to maintain a pH of 7.9. The lower your pH is below 7.9, the more likely you are to suffer a pH crash, resulting in severe stress to your fish. To increase pH, add sodium bicarbonate (baking soda) @ 1 teaspoon per 10 gallons until the desired level is reached. A higher pH alone is not harmful. However, a combination of high pH and ammonia in the water is a deadly combination. Lowering pH can be difficult. Adding peat or vinegar is somewhat effective.
Ammonia comes from fish waste and decomposition of uneaten food. Ammonia levels will quickly rise to lethal levels in a system with new or uncycled filters. Levels need to be maintained as close to 0 ppm as possible. If levels are consistently above 0.5 ppm, you need to decrease stocking density and feeding, and/or increase filtration. A combination of high pH and high ammonia is especially dangerous, because with each 1 point increase in pH (for example from pH 7 to pH 8) the ammonia present becomes 10 times more toxic. You cannot smell or see high levels of ammonia. You MUST use test kits.
Bacteria in your filter system consume ammonia. Nitrite, a toxic byproduct of this process, must also be monitored. Nitrite accumulation will quickly kill fish in a system without fully cycled biofilters. An ideal nitrite level is 0 ppm. Anything above 0.15 ppm is stressful to fish and can cause disease. To reduce high nitrite levels, increase your water changes and filtration, and decrease your feeding and stock density.