This article discusses the ideal water temperature for betta fish

Purchase a heater with a thermostat––Betta fish like water temperatures between 78-82ºF/24-27ºC
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Remember that your Betta is a carnivore (meat eater), so provide himwith a varied and high protein diet. Many Betta foods are availableon the market today, and most are also appropriate. Bettas have smallappetites, so don't overfeed them. Your fish should be able to consumeall the food within two minutes of feeding. If there is food in thetank after this time has elapsed, this will contribute to poor waterquality and make your Betta more susceptible to . You will probably only need to feedyour Betta once a day, though some people feed twice a day. If youfeed twice a day, remember that this will increase the waste your fishis producing and will also increase the risk of over feeding, so itbecomes more important that you are performing your and allnecessary . Providing aproper diet will increase your Betta's life expectancy and bolster hisimmune system.
Since bettas are tropical fish, they require water at a temperature of at least 76 F, and in a range between 76F -82F.
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I hope your betta is ok!!! You need to have a heater, thermometer, and water conditioner for your betta. The ideal water temperature for your betta is 75F - 82F. Please do Not put your fish in a warm cup of tap water to warm it up. Warm tap water can be over 90F which can harm or kill your fish. Remove your fish along with some of his tank water in a large cup at least the size of the container you bought him in. Clean the tank, put the heater in the clean water and put the conditioner drops in which help remove the chlorine from tap water which can harm your fish. Follow the instructions on the betta water conditioner so you know how many drops to put in for your size tank, Make sure the thermometer says between 75F - 82F before you put your fish back in his tank. You can just add the water from his cup to the tank as well. Keeping the same water temperature after cleaning is very important. The heater will keep it consistent, and the thermometer will let you know for sure. Good luck!!! Here are a few important points to remember when keeping water temperature consistent for your betta fish.
Photo provided by FlickrBettas are tropical fish. The normal water temp range should be between 76-82 degrees. Under that and they're prone to digestive problems and fin rot.
Photo provided by Flickrbetta fish water temp?
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BETTA HEATER
Being Tropical Fish, Bettas need to be kept at a temperature of 78 degrees F (25 degrees C) at a MINIMUM on a consistent basis. It would be much better to aim for 80 degrees F (26 degrees C) to make them even happier. This is very important as the Betta seems to be especially prone to some of the cold water conditions such as Ich and Finrot. These diseases are primarily prevented by keeping the tank temperature at steady and proper levels. and water parameters good.The temperature of the water your betta fish is in is very important because they come from a warm and tropical climate, mainly Thailand and Burma. This means that they are used to being in warm waters, preferably between 78 and 80 degrees Fahrenheit. These fish come from cplaces where the climate is very steady and temperatures do not fluctuate all that much, especially water temperatures, so this means that betta fish have gotten very accustomed to a specific temperature range. Maintain the temperature of the water in the tank at 76-82 degrees Fahrenheit. Fish can tolerate moderate temperature changes without any adverse effects. Avoid dramatic, rapid changes in the temperature of the water so it is never consistently too low or too high. Although your Betta can tolerate cooler water temperatures, this environment is not optimum for their health and can cause them to .The ideal temperature for your betta tank is very strict and it is between 78 and 80 degrees Fahrenheit, or 25.5 and 26.5 Celsius. Now keep in mind, this is the ideal water temperature in which betta fish will thrive and do the best they can. Betta fish can survive in waters between 72 and 86 degrees Fahrenheit, or 22.2 to 30 degrees Celsius, but they will only be surviving instead of thriving.