Free chlorine is a halogen chemical element. Concentrated levels of chlorine kill fish and other aquatic life-forms. It is an oxidizer which removes the necessary slime coat from fish, causing stress and stress-related illnesses. As the tank’s pH levels go down, chlorine becomes more toxic. Exposure to high levels of chlorine damages the gill structure of the fish which leads to difficulty in breathing causing them to swim erratically or gasp at the water surface to get enough oxygen. These symptoms can easily be mistaken for low-oxygen problems, gill parasites or other diseases when tap water is to blame. A chlorine level of 0 ppm is ideal.
The carbonate hardness scale is based on the concentration of carbonate and bi-carbonate and reflects the buffering capacity of the water. Water with a high buffering capacity resists changes in pH, resulting to a stable water condition which is essential for the maintenance of many species of fish that cannot tolerate even small pH changes, such as Tanganyikan cichlids. Excessively low carbonate hardness level is not good for aquarium plants, especially those that extract some of the carbon used for photosynthesis from the carbonate and bicarbonate salts in the water.
Below is a guide of desired GH for different types of fish.