I have a 3g tank with just Ghoul in it (Betta male). The tank harbours (apart from the fish) gravel substrate, one live plant (Anubias nana), a piece of driftwood and a PVC cave. No filter whatsoever.
I bought a heater yesterday and set it to around 78. I left home for a few hours and when I came back the thermometer said the water was at about 83!
Now, what I was wondering is, could this be due to a faulty heater and/or thermostat, or does it get inaccurate readings because the water is not "moving" at all (lack of filter)?
(all this assuming that my thermometer is in good working order, I may grab another one just to be sure)