Temperature for Betta fish bowl

I bought this heater for a one gallon fish bowl at work that houses the betta. The temperature of the bowl was 21 degrees Celsius, which is danger zone cold temperature for Bettas. When I put this heater in and turned it on it slowly raised the temperature by about a degree every hour. That is not the issue. It is supposed to stop raiding the temperature when the temperature hits about 28 degrees Celsius. That did not happen, this heater just kept heating and heating.To avoid cooking my Betta, I decided to put the heater on a timer to run for 12 hours only and I suggest you do the same if you plan to get one. Grant it that temperature fluctuations may not be the best thing for bettas, but that is better than exposing him to dangerously cold temperatures. It is bestif you have space to get at least a 3 gallon tank. I have three bettas at home, 2 are housed in a 3 gallon and 1 in a 5 gallon. The tanks are not heated but the ambient room temperature keeps the tanks between 26 and 30 degrees Celsius, The fish are active, colorful and appear as happy as can be. I regret that I did not have the space for anything larger than a one gallon fish bowl at work. If I did, I would have opted for a 3 gallon tank. Again if you get this heater, I suggest putting it on a timer to run for 12 hours at most, so you don't overheat your tank and cook your betta.