Here's why:
* Beneficial bacteria: Goldfish produce waste that needs to be broken down by beneficial bacteria. These bacteria live in the filter and throughout the tank. Completely clean water would mean no bacteria, leaving the goldfish vulnerable to ammonia poisoning from their own waste.
* pH shock: The sudden change in water chemistry (pH, hardness, etc.) can be a shock to the fish, even if the water is clean.
Instead of using completely clean water, you should:
* Perform partial water changes: Replace about 25% of the water every week with dechlorinated water that's the same temperature as the tank. This keeps the water clean while maintaining beneficial bacteria levels.
* Use a water conditioner: Add a water conditioner to your tank to neutralize chlorine and chloramine, which are harmful to fish.
When adding new fish to a tank:
* Acclimate them slowly: Float the bag containing the new fish in the tank for 15-30 minutes to allow the temperature to equalize. Then, add small amounts of tank water to the bag over an hour. This helps them adjust to the water chemistry gradually.
Remember: Maintaining good water quality is crucial for the health of your goldfish. Regular water changes and proper filtration are essential.