My husband came home with a heater for my 10 gallon betta tank. Its an 100 watt and it says its ideal for aquariums up to 25 gallons. Now im worried that its TOO much. He said he got me the bigger one in case I ever decided I wanted it on my bigger tank.
it probably wasn't. good heaters like ebo-jager and visitherm tend to only go up $2-$3 per 50 watts, even at an LFS, so it's likely not worth the time and effort.
You'll probably find yourself wanting a 20gallon before too long anyways
Former advisor and planted tank geek...life's moved on though.