hi, I have a 20 gallon tank and I'm thinking of getting a pair of clowns... My ideal temp is 78f... The thermometer says its 78f but the temp goes warm al of a sudden then goes cold after 10min... I don't get it because my heater is always on until it reaches the right temp, but the water goes cold and warm... Please help(I have my ceiling fan on all the time, maybe that is why? I live in the north, so maybe that is why what can I do to keep the tank warm? Winter is coming soon and the weather is getting cold now... I have my windows open too so maybe it's that... Any suggestions?