Yes, you will need a 25 watt heater. The adjustable, submersible ones are the best, IMO.
You will need to set it so the water stays between 78 - 82 degrees.
Here is one I recently got off Ebay and I am very happy with it:
There are other brands out there as well that are very good - such as Marineland (have some of those also & they work well) - but this one is a good price even with the shipping, IMO, and the shipping from that seller was fast.
Hope that helps & I'm sure you will get plenty of other recommendations! :)