The water temperature is controlled by two separate things. It will seldom be much cooler than the air temperature, even though evaporation cools it a little. The other thing is the heater, which can bring the temperature higher than the air temperature. The setting on most heaters is controlled by a small knob located on the top of the heater. You turn it left or right to change the heater setting. Some heaters have a scale to tell you the temperature setting while others make you experiment with turning the knob and waiting for the tank temperature to respond. If you are not sure the heater is causing the temperature that you are measuring, try unplugging the heater for a day or two. If the tank cools down, it is the heater bringing the temperature up.