Here's why:
* Chlorine and Chloramine: Tap water contains chlorine and chloramine, chemicals added to kill bacteria. These chemicals are toxic to fish.
* Other Chemicals: Tap water can also contain other harmful substances like heavy metals, pesticides, or minerals in high concentrations.
* pH and Hardness: Tap water often has a different pH and hardness level than what's suitable for most fish. Sudden changes in these parameters can stress and harm your fish.
What to do instead:
1. Condition the water: Use a water conditioner specifically designed for aquariums to remove chlorine, chloramine, and other harmful substances.
2. Let tap water sit out: Allow tap water to sit in an open container for 24 hours to allow chlorine to dissipate. This is not as effective as using a conditioner, but it can help a little.
3. Use dechlorinated water: Some pet stores and aquariums sell dechlorinated water. This is a safe and convenient option.
4. Test your water: Before adding any fish, test the water parameters (pH, ammonia, nitrite, nitrate, hardness) to ensure they are suitable for the type of fish you are keeping.
Important Note: Always follow the instructions on the water conditioner product you are using.