So, my heater broke two days ago so as a result, I ended up going to Petsmart to buy a new heater. I got the Top Fin 25 watt which says it's suitable for up to 5 gallon tanks as I have a 3 gallon. I don't know if I made the mistake of getting a 25 watt, but I rinsed the heater, placed it in the tank (& added water conditioner incase there was some chlorine or chemicals lurking on the heater) & it isn't preset, you can adjust the temperature.
So I adjusted it to about 78 and within 2 hours the heater automatically shut off meaning the water was now 78. It is now the next day and my betta is acting a little funny.
Did I shock him? Did the water temperature change too fast?
Not sure which heather that is, do you have a picture?
It doesn't matter what tank size you have or what type of heater ALWAYS monitor the temperature with a thermometer.
This is what allows you to have a strong heater in a small tank or a low watt heather in a big tank... you adjust accordingly.
How did you adjust your fish to the new water?