Heater For A 5 Gallon Tank?

[mikkean]

I'm in the process of cycling a new 5 gallon tank for my daughter's betta that is currently hanging out in the bowl she bought him. When we got the tank, we had an adjustable 25w heater in the cart and the store employee said it was too much and that we should get a 10w for that small of tank, so I did. The 10w heater is an Aqueon non-adjustable and while it does heat the tank, it varys the temp between 80 and 84 - I thought it was steady at 82, but have seen it increase to 84 on the warmer days. Is there a better adjustable heater that I should get to keep it at a steady temp but will work with the small tank since I thought I read someplace on here that highest for bettas should be 82?

Thanks!