How can I change a fish's water effectively and safely?

To change a fish's water effectively and safely, follow these steps:

1. Prepare new water by using a water conditioner to remove chlorine and other harmful chemicals.
2. Use a siphon or aquarium vacuum to remove about 25-50 of the old water.
3. Gently refill the tank with the new, conditioned water at the same temperature as the old water.
4. Monitor the fish for any signs of stress and adjust water changes accordingly.