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One of the primary factors in keeping your fish healthy is creating an environment that suit for different kind of fish. Since fish reside in a wide range of water habitats such as fast-flowing mountain streams, great lakes, to slow-moving rainforest rivers, it is good to have some knowledge of basic water chemistry, specifically ph, water hardness, and the nitrogen cycle.


In aquaria, the pH of tap water can be manipulated though the use of phosphate, bicarbonate, and other buffers. However the pH may change despite these buffers. The development of a low pH can usually be attributed to decaying material such as plant and animal wastes. When the pH drops below 5.5, bacteria (Nitrosomonas) that break down ammonium are reduced, and the ammonium level rises.

  • Lowering the pH: The pH can be lowered by using a pH-lowering chemical (available at pet shops). Be aware that many pH-lowering products use phosphate-based chemicals. Phosphate is a nutrient that encourages algae growth. The pH can also be lowered by plants and fish during respiration, when CO2 is exhaled into the water. In hard water, the pH may be difficult to lower. First, soften the water by peat filtration or reverse osmosis, and then try lowering the pH.
  • Raising the pH: The pH can be raised in an aquarium by adding baking soda (bicarbonate) or making a water change (thus removing some organic buildup which reduces pH).

Water Hardness

The degree of water hardness relates to the amount of dissolved minerals, especially calcium and magnesium, in the water. Water hardness is generally expressed in the amount of calcium carbonate (CaCO3). Water hardness is measured in ppm (part per million), kH (carbonate hardness), and dH (degrees of hardness) or gH (general hardness). Water is expressed as soft (having few dissolved minerals) or “hard” (having many dissolved minerals. General levels of water hardness are expressed in the table below (1 gpg=17.1ppm=17.1mg/l).

  • Soft Water- less than 1 gpg
  • Slightly Hard- 1-3.5 gpg
  • Moderately Hard- 3.5-7 gpg
  • Very Hard- 7-10 gpg
  • Extremely Hard- over 10 gpg

For most aquariasts water hardness is not particularly important. However, excessively soft water can cause problems. Because softer water has less buffering capacity due to a lack of CaCO3 (a natural buffer), and is subject to pH crashes, where the pH falls dramatically causing harm to aquarium inhabitants.

  • Soften the water: The water can be filtered through peat moss or filtered through a reverse osmosis system. Ion exchange resins also can be used to lower the water hardness. Boiling water for a period of time can also reduce its hardness.
  • Harden the water: Filter the water through dolomite or crushed coral until the desired hardness is reached.

By running an electrical current through the water, the level of conductivity can be found. Conductivity indicates the amount of ions (electrically charged particles) is in the water. The higher the water hardness, the greater the conductivity. Testing the conductivity of the water only finds the total amount of ions present in the water, and does not give the origin of the ions, whether they are Mg, Ca, or Fe.

The Nitrogen Cycle

When fishes excrete waste, plants deteriorate, and food rots, the resulting waste does not just disappear. The waste, in the form of nitrogen compounds, is broken down into other compounds by bacteria, until the final product, nitrate NO3 is absorbed by plants or removed through gravel siphoning. Nitrogen waste products become toxic to fish and other aquarium inhabitants at high concentrations.

Quote Left A well-balanced tank contains organisms that are able to metabolize the waste products of other aquarium residents. Quote Right

Nitrogen cycle occurs when the tank becomes out of balance. If waste levels become too high, because of over-feeding, overstocking, and/or lack of water changes, ammonia levels will rise again. In this case, water changes should be performed until normal levels are restored. A well-balanced tank contains organisms that are able to metabolize the waste products of other aquarium residents. The nitrogen waste produced in a tank is metabolized in aquaria by a type of baacteria known as nitrifiers (genus Nitrosomonas). Nitrifying bacteria capture ammonia from the water and metabolize it to produce nitrite. Nitrite is also highly toxic to fish in high concentrations. Another type of bacteria, genus Nitrospira, converts nitrite into nitrate, a less toxic substance to aquarium inhabitants. This problem is most often addressed through two filtration solutions: Activated crabon filters absorb nitrogen compounds and other toxins from the water, while biological filters provide a medium specially designed for colonization by the desired nitrifying bacteria.

References: Wikipedia

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