So when I first set up my tank I had gotten Tetra's submersible heater for 2-10 gallons for my 2.5 gal aquarium.
I've heard a lot of bad things about this heater - that it stops heating and basically becomes useless. It's supposed to keep water at 78 degrees +/- 2 degrees. This was my experience the first time around. I left it in the tank during my water changes (usually unplugged it first) but after a while I noticed it didn't keep keep my tank anywhere near 78 degrees. It would fluxuate between 75 to almost 81. Bad for betta.
I realized that this heater is very, very finicky. It can't experience any temperature shock or the thermostat will die. The directions said to unplug it for at least 15 minutes before taking it out of an aquarium. It also said to put it in an aquarium for 15 min before plugging it in. And NEVER leave it on an expose it to open air.
Now I'm new to the betta hobby and didn't realize that when the company gives you these instructions - you really NEED to take them.
So I bought a new heater (same kind, its the only one for small tanks at my lfs) and followed all the rules to the letter. Unplug it for 15 min before water change. Leave it in during water change and wait 15 min to plug it back in. Since doing that it's been working great! It's been keeping my tank at a comfy 78-79 degrees and my betta has perked up.
So, in my experience anyway, this heater is great for small aquariums if and only if you use it properly and don't blow the thermostat. It's a picky device and can easily be broken. :/