I don't have the smaller heaters available here and they were out yesterday in the city =P But I have the same heaters as you in my 2.5 and 3g and there hasn't been any terrible fluctuation in temperature. So maybe a123andpoof is right and the tetra heaters are crap. Which is fine by me since they haven't cooked my fish yet.
Btw, nice tank and grats on otis =) The only thing I might suggest is a small cave maybe, just something more solid of a hiding place. People use coffee mugs or small pottery pots if you don't have any decorations you like at the stores.
What is the deal with filters? As you can see, I am using the Whisper that came with my 3 gallon tank setup. I hear that bettas dont necessarily like ANY current. Otis seems to have fun though sometimes swimming against the current then back again on the right side of the tank.
Is that not good for them? I am going to pick up something better for him to hide in, per some suggestions on the tank thread. Also going to get the plant you suggested to replace the fake purplish colored one in the center.
I use tetra in all my tanks. but am working on changing them out of my larger ones. because they have trouble heating ten gallons. 5 gallons is even pushing it *rolls eyes* but the 15 w works great in anything 2 gallons and under!