Sick Betta

If you want him to live, you are going to have to get a test kit and make sure that the ammonia and nitrite are below .25, and your nitrate below .40, even if you have to do multiple water changes a day. He also needs his water to be heated and consistent. Makes sure when you do your water changes that your water is the same temperature as the water in his tank, and treated to remove chlorine, chlorinates, ammonia, etc. You can use Prime to do this.
 
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