I just read in a thread on the mother forum (Tropical Fish Keeping) a suggestion to install submersible heaters horizontally to have better temperature control. Supposedly most heaters have their sensor in the upper part and by placing it vertically the reading might be slightly off, which might contribute to false readings and fluctuation in temperature.
Can anyone second that here in the Betta forum? I have two submersible heaters and when they were set to 78 to 80 degree the temp always went way up during the day. The setting is now just over 76 and seems to hold the temp pretty good at 78 to 82 in the course of the day.
If installing the heaters horizontally really is a better way of providing a stable environment, I will change the direction tomorrow during the weekly maintenance.