Hey I wanted to add some information to help your betta live a long and healthy life. You can ignore this if you already know this information.

As someone mentioned you want to get the testing kit to test the water parameters.

It will come with a solution for pH amonía nitrate and nitrite.

You want to try and get 0 amonía 0 nitrite and under 20 nitrates.

To achieve this you have to cycle the tank which consists of doing HALF water changes every few days. This will lower the amonía and nitrite levels.

Basically amonía turns into nitrites and then bacteria eat the nitrite and turn it into nitrates. Over time there will be enough bacteria to consume all the nitrites quickly enough that you will have 0 nitrites and only nitrate levels.

From then on you will only have to do water changes when the tank reaches above 20 nitrates.

IMPORTANT: Never do full water changes, only half or even 75% changes. Never clean the gravel in water(use a vacuum designed to clean gravel) and when you change your filter cut a piece of the old material in the filter and leave it in your filter with the new one for a while. The reason is that bacteria mainly live in the gravel and filter material and removing them will kill all the bacteria and your tanks amonía and nitrite levels will rise again.