Tap water, aged several days to release the chlorine, is adequate. If you have a high level of minerals in your tap water, you should filter it through some charcoal (available at pet shops as "Activated Filter Carbon". You can ask about the local water quality at a pet shop.

If you use a filter in your tank, replace half of your water once a week. If you are not using a filter, replace 1/3 of the water each day. Always siphon water from the bottom of the tank to remove uneaten food and waste.

The pH level should be kept around 7.0 with 6.6-7.4 the maximum. pH kits can be purchased from any pet shop. Some municipal water supplies are raised to 8.0 or more to reduce leaching of copper, etc. from the pipes. You may want to phone a local pet shop to see if you will need to make any initial adjustments to your fresh replacement water.

Note:  Water that has been made suitable for the environment of the fish will be referred to as "system" water.