Always make sure that the bowl or tank that your betta fish are in has enough breathing room (the surface). This helps the betta fish breath. Also, make sure its large enough to accommodate him or her. You don’t want your betta fish ripping or tearing a fin.
If you’re not using a filteration tank, then you should be cleaning out his water every 3 days or so. The best water for them if water that is still and has been left out for at least 24 hours. This is the water that is meant to be used when replacing the old one.
Never, ever put betta fish together. They are called siamese fighting fish for a reason. They will often kill each other, and another fish before they’re done. You can couple them with algae eating fish and catch fish. This will make for great tank mates.