Summarize:

The most common cause of popeye is dirty water, and the best form of prevention is frequently replacing the dirty tank water with fresh water. Always have clean water in the betta’s fish tank or bowl to ensure that it won’t get popeye.  If your betta is kept in a bowl or tank that holds two gallons of water or less, do a 50% water change every week. If your betta is kept in a larger aquarium, change 10-25% of the total water every 2–4 weeks. Clean the tank every week if you don’t have a filter and every two weeks if you do.  Gently remove your betta fish with a net and place it into a separate clean bowl of water. Dump out all of the water in the fish tank, remove all rocks and decorations, and rinse them with clean water. Using a paper towel, scrub the inside walls of the tank. Put the rocks and decorations back in the tank and fill the tank most of the way with bottled drinking water or pretreated tap water before putting your betta fish back in. The natural habitat for betta fish is warm, stagnant water. Make sure the water is kept between 76 and 82 degrees Fahrenheit (24.4-27.7 degrees Celsius) in order to provide a healthy environment for your fish. Use paper pH test strips to test how basic or acidic your betta’s water is. The pH should be kept at 6.5 or 7.  If the pH is too high, filter water through peat moss before putting it in the tank. If the pH is too low, add baking soda or shells to the tank. Bettas prefer soft water, so keep the dH at 25 or lower. Go to a pet store to get specialized products that can extract the magnesium and calcium from your water if it is too hard. Different fish require different environments, so make sure that you don’t add any fish to your tank that have conflicting environmental needs. Popeye often occurs when the tank water is not kept at the proper levels, and adding a new fish that thrives in a different kind of ecosystem might throw off those levels.
Change the water regularly. Clean your fish tank every 1–2 weeks. Keep the tank water warm. Keep the tank water slightly acidic. Purchase a dH kit to determine water hardness. Carefully introduce new fish to the fish tank.