I'm going to have to disagree with the idea that neons are a good idea to house with bettas, for a couple of reasons. The first reason is that they are very likely to nip the long fins on the betta, this is the less important reason. The second reason is that they just aren't temperature compatible. You CAN house neons with a betta, but no matter what temperature you keep your tank at it will always be at the expense of one of the two species. While bettas need to be in the 80-82 degree Fahrenheit range, neons should be kept at 72-75 degrees Fahrenheit. Keeping the two species together almost guarantees problems with the health of your tank regardless of aggression, because no matter what you do one of the species will be pretty far out of their appropriate temperature range, substantially raising the chance of disease and infections.