I have a single betta living in a 2.5G heated tank, heated with a Hydor Theo heater. The water temp is typically 80F but yesterday I noticed it had crept up to 82 or 83 prior to the scheduled 100% water change. I did the water change with 82F water, not wanting to cause any sort of temperature shock, but turned the heater down when I put it back in the tank. Earlier today the temp was up to 84F so I turned the heater down another click and now the water temp is at 85, the heater setting is all the way down at 76 and the heater is still ON (it is supposed to auto shutoff when desired temp is reached).
What should I do? It's really cold here so going without the heater isn't an option. I don't have ready access to a new heater - I had to mail order this one and it took weeks to come. I do have another old Hydor Theo but it was in a tank with a fish who died mysteriously so I'm hesitant to re-use it. How can I best get the water temp back under control without stressing my poor fish (who seems quite happy at the moment, I'm just worried it's going to keep getting hotter and then he won't be so happy)
ETA: it's not due to water evaporation - since doing the 100% water change yesterday the water level hasn't changed noticeably.