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Water for Aquariums

Providing water of appropriate physical and chemical quality is probably the most important factor for the care and maintenance of captive fish. The water quality and temperature should be optimized for the specific species of fish being used.

The following water contaminants should be avoided when preparing water for fish tanks:

  • Chlorine and Chloramine
    Chlorine is highly toxic to fish even at levels as low as 0.1 ppm. Chloramine is even more toxic than chlorine because it is more stable in water. Chloramine penetrates in the blood of fish and binds to the iron of the hemoglobin in red blood cells, causing a reduction in the cells capacity to carry oxygen. Activated Carbon, placed up-stream of the reverse osmosis membrane of water purification systems is known to remove efficiently these oxidizing agents.
  • Ammonia, Nitrites, Nitrates, Phosphate
    Ammonia is toxic in both freshwater and saltwater tanks. Its toxicity increases with temperature and pH. Even low levels of ammonia can stress fish and make them vulnerable to diseases. Nitrite is not as toxic as ammonia, but can stress the fish as well. Nitrite may not have readily visible effects but it interferes with the oxygen metabolism and can affect the physiology of the fish (and therefore research results). Nitrate is less toxic than ammonia and nitrite, but it can also cause stress in fish and should be kept at a low level. Phosphate levels should be carefully controlled, as it will lead to algae growth. Algae consume oxygen, lowering the oxygen level in the tank. Using reverse osmosis decreases the overall ionic concentration of tap water and ensures suitable ionic concentrations.

  • Heavy metals
    Heavy metals (copper, iron, zinc, lead …) can be toxic to fish, and should be as low as possible.

  • Variations in pH
    Fish are very sensitive to variations in pH. A pH above 8 may prevent the growth of denitrifying bacteria, leading to the accumulation of toxic nitrogen compounds in the water. Low pH (below 6) may have toxic effects on fish. By reducing the levels of ions and organic acids present in water as much as possible, one can keep the pH of water in the vicinity of 7.

  • Hardness
    Depending on the fish species, different levels of water hardness may be required. Water hadness is important to fish health because it affects osmoregulation. Hardness must be controlled because it also has an effect on pH and pH stability. Anti-scaling agents are usually present up-stream of the reverse-osmosis membrane of water purification systems, thereby ensuring low levels of hardness.

  • Organics
    Some organics, such as pesticides, endocrine disrupters, etc. which may be present in tap water, may affect the health of the fish. Organics can also be used as nutrients by bacteria. In turn, the proliferation of bacteria will lower the oxygen level in the tank Organics can also affect spawning. Most organic molecules are efficiently removed by activated carbon and reverse osmosis.

  • Bacteria
    While some bacteria are beneficial to fish tanks because they eliminate nitrogen waste, bacterial levels in the make-up water should be kept at a low level. Bacteria can consume large amounts of oxygen, thereby depleting oxygen levels in the tank. In addition, some bacteria may affect the health of the fish.

    Flow-through fish tanks require fleshly prepared water. Recirculating water systems require make-up water to replace the water lost by evaporation, and also replace about 10 to 15% of the tank water. It is important to keep a constant water quality in fish tanks. In addition, fish raised in one water quality tend to do poorly when transferred to a different water quality. It is not recommended to use tap water because it may contain some or all the contaminants listed above, and it may vary with the season or the location. In conclusion, water purified by reverse osmosis is usually recommended for this application.


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