Last night I switched my betta to a 5 gallon tank. He has previously been in a 3. I put the 25 watt heater in there last night and the thermometer said 78 so I knew the temp was rising which was good! then this morning I woke up and the tank temperature was like 68! The heater is 25 watts it is not submersed in the water in hangs on the side and it has a round dial on top that goes low ------> high It is right before high! So I threw that on a little higher and put it my mini pad heater just to raise the temp! Is this heater not working? The top of the tank is warmer than the bottom. I do have an ac in my room though it was on 65 all night was that the whole problem?