My Top Fin aquarium heater has been working fine for many months until now. My tank water is slowly decreasing temp but heater says it is on. The temperature outside has dropped big time and it is snowing a ton. I think I need to increase the temperature on the heater but I don't know how to do this on this certain Topfin heater there is no arrow pointing to which way I should turn to increase temperature it is just a knob that you can turn. Any suggestions?