Chlorine and chloramines
Disinfectants present in tap water. Deadly for fish, invertebrates and filter bacteria. They must always be neutralized before use.
Detailed explanation
Drinking water contains chlorine (Cl2) or chloramines (NH2Cl) added to eliminate pathogenic bacteria. What protects human health is lethal for fish.
Chlorine destroys the gills of fish causing suffocation. Kills the bacteria in the biological filter by resetting the nitrogen cycle. Even low concentrations (0.1 mg/l) are dangerous.
Free chlorine evaporates from water if allowed to stand for 24 hours with stirring. Chloramines, on the other hand, are much more stable and do not evaporate. In many Italian cities, chloramines are now used instead of chlorine.
Water conditioners (such as Seachem Prime, API Tap Water Conditioner, JBL Biotopol) instantly neutralize both chlorine and chloramines. They should be added to tap water before using it for water changes. Seachem Prime is particularly popular because it also neutralizes ammonia released by the decomposition of chloramines.
Reverse osmosis water does not contain chlorine or chloramines because they are removed by the activated carbon stage of the system. For those who use osmosis water the problem does not exist.
NEVER rinse the filter material under the tap. Chlorine kills bacteria in seconds. Always use aquarium water for rinsing.
Particular attention must be paid to ponds that are filled with a garden hose. A slow filling with water conditioner added in proportion is the safest method.
How it is measured
Drop test for free chlorine (swimming pool). In aquarium practice, the water conditioner is always used without testing, for safety.
What happens when it is too high
Lethal. Fish that gasp and die in a few hours. Filter bacteria destroyed. Dead invertebrates. Immediately add water conditioner and increase ventilation.