What Ajones has would probably be ideal. I don't know for sure, but from what I've seen, alot of people have problems with tetra heaters (These are the ones that can be found commonly at Wal-mart). In a fish-guide video, its pritty clearly state that, "You get what you pay for", I believe this to be true. What ever heater you do look at, just make sure its for the correct tank size. This will be displayed clearly on the box before you buy it.
As state already, Temp should stay around 78-82 for good comfort, being at 76 wont hurt though. Keep in mind that these are tropical fish though, so I wouldn't go lower. I hope this helps a little!