As a general rule, it's recommended to change the water in your betta tank every week or so, for about 7 to 10 days. This will help keep the tank clean and prevent waste from building up. The amount of water you should change depends on the size of the tank. For a small tank, you should change between 30 and 50% of the water every week.
For a larger tank with a filter, you should change between 10 and 20% of the water at least every 7 to 10 days. Before you start changing the water, make sure you have all the necessary equipment in place. A sponge filter can provide your tank with enough filtration for a betta and several tank mates. A siphon hose for cleaning the tank allows you to get the water out of your aquarium safely.
You'll also need to consider heating the water and measuring it with a tank thermometer before adding it to your tank. Bettas thrive best at temperatures between 75 and 80 degrees Fahrenheit. When you add fresh water, you will have to do your best to match the chemical and temperature parameters, since a change that is too rapid can cause diseases and other problems in fish. To make sure your betta fish is healthy, use a practical kit to determine the chemical composition of the water in your fish tank.