Sometimes the cleaning of all filters at once, or the changing of the gravel can trigger a bacteria bloom, due to the removal of bacterial colonies that had 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 are feasting on high nutrients. In nutrient rich water they can multiply at such a high rate that the water becomes cloudy white. 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.
Once again the focus should be on nutrient control . Eliminating excess nutrients will starve the unneeded bacteria colonies resulting in a clearing of the water. A UV-Filter may be somewhat effective, as it kills bacteria. Nevertheless, the problem of high nutrients will not be fixed this way. Therefore the nutrients have to be lowered through water changes and the use of products such as Algone .