If you have just started your aquarium, or recently changed the gravel and are experiencing a grayish discoloration of the aquarium water, don't panic. The gravel was probably still dirty. The free-floating dirt particles should settle and get trapped in the external filtration fairly quickly.
A light haze after siphoning the gravel is harmless, and is likely caused by the minor disturbance of the substrate. This usually clears up within 24 hours.
A yellow discoloration or brown tint is typically attributed to high levels of dissolved organic matter. This can cause the pH to drop significantly, posing harm to the health of your aquarium fish.
The green water that affects visibility, and is often referred to as pea soup, is an algae bloom— free-floating planktonic single-celled algae growing at a rate that turns the water green.
The cause is always the same— too much light and poor water quality. Excessive light can be caused by the aquarium lighting, as well as intense room lighting and direct sunlight.
Water changes provide very little help in clearing the water. Algae spores are readily available in the water, including most waters used for changes. An algae bloom can become so severe that the content of your aquarium can no longer be seen in the green water.
Some believe that turning off the lights will eliminate the problem, but this is an ineffective solution, and the problem will continue to occur.
Algae will consume oxygen at night during photosynthesis. A severe algae bloom can deplete the tank of oxygen, so adequate oxygenation must be provided during the light off period.
Dying organic matter creates phosphates, so the filter should be rinsed more frequently during an algae bloom, eliminating some of the decaying matter. Vacuuming the gravel will also help.
White cloudy water is a result of a bacteria bloom.
Sometimes the cleaning of all the filters at once, or the changing of the gravel can trigger a bacteria bloom, due to the removal of bacterial colonies that have settled on the filter media or substrate. Another cause can be medical treatment of the tank using antibiotics, which may destroy these colonies.
The bacteria are either re-establishing themselves, or crowding the aquarium due to favorable conditions (poor water quality), multiplying at such a high rate that the water becomes cloudy white.
A bacteria bloom is cause for concern:
Bacteria need oxygen. A few grams of bacteria consume about the same amount as an adult human, again posing a threat of de-oxygenation in the aquarium. Immediate action is required if the problem is severe, or persists.
It is also advisable to check on ammonia during the period of a bacteria bloom, as ammonia may rise to dangerous levels.
In a salt-water aquarium, the protein skimmer usually helps to prevent and cure a bacteria bloom, as the bacteria are removed as particles. Make sure the skimmer works properly when a bacteria bloom arises.
A severe bloom can create an oily film and enough foam-depressing agents to make a skimmer go flat (no foam, no function). This has to be taken very seriously. Despite oxygen depletion, ammonia gets more toxic at higher pH levels, which is characteristic for salt-water environments.
It is therefore vital to make the skimmer work again. An oxidation agent such as potassium permanganate can be used. Diluted and used sparingly (1% and only added in the low ml region), it will oxidize enough fat to be picked up by the skimmer, thus jump-starting the system.