Keep the container clean
Use a food-safe or aquarium-purpose container that has never held soap, cleaners, or chemicals. Label it so it is not reused for household tasks.
If you store water, keep it covered from dust while allowing the preparation method to work as intended. Do not assume stored water remains identical indefinitely.
Prepare each batch deliberately
Condition tap water according to the product label, then test hardness and temperature as needed. Remineralize purified water in the container rather than in the display aquarium.
Record the source and readings. If the tap changes seasonally, test it again before a large water change.
For your own tank
Label the container with its source and intended use, and never use a bucket that has held detergent. If water is stored, test it again before a large change. Clean containers and consistent preparation reduce the chance of accidental contamination.
Use these as general hobby references. Test your aquarium and confirm product guidance before making changes.