There should be a thermostat on an aquarium heater. What brand is it? There should be some means of controlling the temp.
33 C is like 91F. That is too hot for most tropical fish. Besides, without a temp control, the temp will vary with the ambient temp & other factors, there is no guarantee that you won't be making fish soup.
If the heater is a substrate ribbon heater or similar design, they are not meant to be plugged into an outlet. They are supposed to be used with a temp controller, something like thsi: