When I bought one of those stick-on thermometer strips for my Nano I assumed it was just inaccurate as it always seemed to over read. I have a 10L Nano without a heater. Out of curiosity I checked the temperature with a scientific and an old photographic thermometer that I have and they both agreed with the stick-on one. They all read about 23.7°C which is a surprise as ambient air temperature in the room it’s in is only just over 22°C. A minor mystery then I realised the internal filter must be having a slight heating effect.