I bought a 20 gallon tank and a submersible heater for 10-30 gallon tanks that is preset to 78 degrees. My temperature is 75 :-(. Could this be due to lack of water current? If that is the case would an air stone help and should I place it directly below the heater to disperse the heat? Don't worry that my fish are freezing, I won't have any until next week-end.